Minimum order quantity for the populated board is 2. That costs, iirc, around €20 for both plus shipping (if you are patient, that's a few more €).
This micro soldering part can by now be ordered to be included in the assembly, so that you don't have to solder it. I haven't tried the process of ordering extended parts from JLCPCB, but if that works, that's another ~€10. Otherwise you'll have to source it yourself and end up with a similar price.
The Blackberry keyboards go for €3-6 on Aliexpress.
A 5-pack of the USB connectors is about €10 on Amazon, but on Ali you can get one for ~€1.
That's all the electronics, so for two of them, that's about €40-50.
You also need a 3D print, which costs around €1 if you print it yourself or get a friend to do it. If you order it, that can be anything from €10 to €50. The cost of these services varies wildly. You might need to print it multiple times to get the tolerances exactly right. So if you get access to a printer, that would save a lot of money.
But all in all, if you manage to build two and both work fine, there are lots of people on Lemmy, Reddit or Crackberry who offered me to pay €100 for a ready-made one.