Old keyboards aren't cheaper, just grimy used mechanical keyboards are cheaper. If you don't mind cleaning up an old keyboard you could grab an ancient mechanical keyboard and clean it up, giga has a link for such a procedure. The keyboard giga listed in that build is rubberdome.
Rubberdomes are significantly cheaper to make, rubberdomes require a single rubber membrane with conductive points on the domes with a circuit board lining the bottom, in terms of cost to make them, I'd be surprised if it cost more than $1.50. Mechanical have complicated switches under every single key that is then bound to the circuit board underneath, many additionally have steel backplates, making them near unbreakable.
I just threw in an 8$ logitech keyboard.
I'm gonna make a few trips to good will while this build is in the works and maybe I could find something nice.
If it's really loud it's mechanical, like that's what I go by right?
The way you check if its a mechanical is by removing the key cap, it should have a switch of some kind underneath it. If its really hard to remove the keycap, chances are its a rubber dome, keep pulling and you might break it. Depending on what it is, the switch will look like arounded cube with a depressable stem coming out the top, or possibly it will just have a whole with a spring in it if it is a buckling spring type.
one of the better rubber dome keyboards though