What Item have you purchased for less than $100, that has made a large difference in your life?

A sleep mask. I sleep much better now that all the light is blocked out. Also, if I want to, I can sleep in in the mornings when it's bright outside and the light won't disturb me.
- I bought one with Bluetooth headphones built in. I have tinnitus and going to sleep was torture. I'd lay in the quiet room and be unable to sleep thanks to the loud ringing in my right ear. During the day, my hearing aid helps, but I take it out at night. With the Bluetooth sleep mask, I can put on white noise, drown out the ringing, and fall asleep.
  As a bonus, I made a specific white noise mix for sleep and I seem to have trained my brain to have a Pavlov-type response. When that white noise goes on, my brain says "time to go to sleep" and I'm able to quickly fall asleep instead of having my brain run a mile a minute in the dark. (At least, on most nights. The Pavlov response isn't strong enough to overcome me being stressed about something if I go to bed.)
  The sleep mask with Bluetooth cost me about $30 and it was worth every penny!

Cheap Amazon bidet. $40 and it revolutionized my #2 experience.
- These are magic, you can get good non-electric ones for $25 each and outfit your whole house. Three years from installing my first one I'm planning for power outlets near the toilets and saving for one that will yell at me in Japanese while it cleans.

Electric tooth brush. Learn to use it properly. Combine with flossing, save a lot on dental issues.
Arm sleeves (Korea brand from ebay) for driving. Keep my arms from turning into toast these days, you could also wet them to enhance cooling

- What purifier did you pick up? We got one that was well recommended, and our Air Quality Monitor has basically said it's doing nothing. It's been pretty frustrating.
  
  This is going to sound silly but I’ve heard some people aren’t aware you need to open it and take the plastic off the internal diluted before first use. Did you do that?

A cheap beginner ukulele. Less than $100 gets you a new decent/good quality uke. (And $100 goes very far for a used one.) There is a whole world of music, musical styles, clubs, jams, online communities, and free resources. And uke players are generally kind, funny, and generous. “No big egos with tiny instruments.”

Noise cancelling headphones (or foam earplugs). Realized a good 60% of my anxiety had been caused by there being too many noises.

A bag of chickpea flour. Somebody on Lemmy was singing the praises of chickpea flour plus water as a replacement for egg in breading stuff. So instead of flour, egg, breadcrumbs I can do flour, chickpea flour + water, breadcrumbs. And then the rest of the chickpea flour goes back in the cabinet and I don’t have to try to use eggs before they go bad.

For any other girls, for God's sake get a proper fitted bra. Spend the money. It's a world of difference

I bought an insulated tumbler and now I drink too much water. I put ice in in the morning and it stays cold all day and I keep it by my bed for those 2am thirsty wake ups and it just hits the spot. Whenever I travel without it I just feel parched all day lol.

Had a baby this year and I have a twofer:
I got a used prefold + covers lot for $100. Cloth diapers seem more comfortable for the baby and they are way easier on the wallet.
Woven wraps can be found used for $20-$80. A woven wrap is a baby wrap you can use from birth till they stop wanting to be held (35+ lbs). They are cheaper and more versatile than many structured carriers.
- Go team cloth! We cloth'd both of my kids. Not always fun, but it's better for the environment and a ton cheaper!
- Used cloth for my first, but I'm old now, so my second gets disposables.

A good set of ear plugs. My toddler can be pretty screamy and I find it a lot easier to be a rational parent when I have ear plugs in that don’t muffle all sounds, but just take the edge off the loud ones.

A massage gun. Works so much better at getting knots out than just your hands!
- Which one did you get?
  
  Lifepro Sonic

Here are a few things I haven't seen mentioned but have a significant quality of life improvement for me:
High quality LED flashlight that can also double as a power bank (e.g. Sofirn BLF SP36, BLF LT1, or Q8 Pro)
Curved shower bar -- no more fighting or getting stuck on the shower curtain/liner
Padded mat for in front of the sink, placement depends on where you spent the most time (handwashing dishes, food prep/cooking, etc)
Over the sink dish rack -- Frees up a ton of counter space if you hand wash
"Cloud" foam slippers

Love my Merrels, I wear the mocs which I got because they have a rounded heel (good for driving) and I was always in and out of them getting in and out of the bunk of the truck. They fit really well despite being slip ons.
A good penlight, streamlight makes some decent models, much better than a phone flashlight and comfortably fits in the pockets of loose fitting mens clothing to carry everywhere. usually more useful in the winter if you aren't a night owl.
A decent pocket knife, nothing fancy, but something heavier duty than a little Victorinox is nice to have though those are also great to carry! CRKT , Civivi, and Kershaw make a lot of nice models under $40 that will last you a lifetime if you take care of them. 8Cr13v is bargain bin steel by modern standards but it has some nice qualities, easy to sharpen and mildly rust resistant, holds a much better edge than a 316 "surgical grade" stainless knife from the dollar store for sure. I have found countless uses for a pocketknife over the years and I'm sure you will too!

My old Kindle Keyboard (3rd gen kindle), still about $30 on ebay, I believe, is my absolute favorite way to read long-form information, and I can hold it however is comfortable instead of awkwardly avoiding the screen with my thumb. I read a lot more books because of it.

Battery powered air compressor and jump starter. Both have saved me from being stranded form a dead battery or low tire.

for something that I don't think a lot of people consider...I got a wrist rest for my mouse and keyboard as a present that I had asked for. it's significantly reduced my wrist and hand pain while typing or using my mouse, which is nice, because I already have pain issues.
additionally! my other mention would be my weighted blanket. I got one as a gift (but it wasn't heavy enough, make sure if you get one it's the right weight) and basically tried it and slept better than I ever had. it only sucks during summer because it makes me warmer. but, other than that, I highly recommend one if someone is thinking about it.

A cast iron pan, despite the little more maintenance required, cooking is so much easier and food just taste so much better.
- I have a nice cast iron, but am always scared to use it because I don't want to clean it the wrong way. Do you have any cast iron maintenance tips?
  
  I mean don't worry too much, unless it physically cracks or something you really can't ruin it.
  Don't wash it while is hot, and if you do wash with soap and water make sure to dry it very well and apply a thin coat of oil to avoid any rust build up. A metal spatula is also great to scrap the pan if it's not too messy and then you can just keep it using.

Long fiber optic hdmi cord to hook up PC to TV and a wireless keyboard to navigate the desktop from the couch.

Lap desk for playing PC games on couch
- Explain more? What is that?
  
  Basically a flat piece of wood or plastic that can hold your keyboard and mouse, with some kind of cushion attached underneath so that it comfortably sits on your legs. I got this one last year to help with working from home in a comfy chair and it’s been pretty great (Amazon link)…
  https://a.co/d/dbrEYtP

A noise generator (LectroFan). I live in a noisy area (Friday/Saturday nights) and the machine completely blocks out all sound. It is more comfortable for me than wearing earplugs or headphones.
Similarly noise-cancelling headphones made a HUGE difference to my life but they're a bit more than $100. Still recommended for anyone thinking about it.
- I gave my mom a set of noise canceling aiwa's i got for free through work. Looks like they only list for $80 but 2 or so years on and she uses them every morning when shes exercising and absolutly loves them. Sometimes cheap chineseium stuff isnt as disposable as it looks
  
  My Sony WH1000-XM3s started turning themselves off after a few years. Their noise cancelling is probably better than others but I can't say they've been reliable.

A foam mattress topper. Really was a lifesaver for a while when I couldn't get the whole mattress replaced.
Also, a tire pressure gauge. Had to use that thing more times than I could count through the years.