Unfortunately, AACOMAS doesn't care how much you spend on pre-reqs. Look at it as a major favor that we've got grade replacement at all! That said, no, you don't have to take it at the original institution. You just have to make sure whatever institution you take it at offers the same class (incl quarter/semester format) at the same or greater number of credits. You can't replace a 4-credit class with a 3-credit class, just as you can't replace a 4-credit semester class with a 5-credit quarter class. Check out online courses as well. You can sometimes find much cheaper classes that way, especially if they're not core sciences (which probably look better at regular old 4 year institutions).