I don't even know why it does this, tbh, since pretty much everyone writes $xx.xx and not anything else, such as xx.xxUS$ or anything like that.
Note: Turning off Auto-Punctuate does not fix this problem, so this is my hail mary attempt to get rid of this "feature." :P
(Edit: My phone is a Motorola Droid 2 Global. The problem exists no matter if the physical or on-screen keyboard is used.)