I haven't been keeping up with keycap prices lately but I know back then it used to be something like $60-70 for a full set. So I'm not sure if you can find something less than $30 but otherwise 1.5 mm is currently the thickest keycap thickness.
I think any thicker it will interfere with keycap function (clearance issue).
Also keep in mind in general thick keycaps doesn't play well with Costar stabilizers but it will be completely fine on Cherry stabilizers. And even between thick PBT different manufacturer will make it differently and it will sound slightly different.
Good luck and let us know what you end up with.