Buy 1 or 2 sets of test prep books, go through the whole thing. Then buy and take every official practice test on the AAMC website. Since they're old tests, they're noticeably more accurate than any third party tests. In fact, I wouldn't waste my time with practice tests on third party websites. Half the time, they test random stuff to scare you into buying their product. That said, money shouldn't be a factor in your test prep. You are going to be spending 15-50k a year on tuition alone, test prep is pennies compared to that.