That would depend on the level of expertise you need. To pass a highschool calc exam, probably less than 100$. To have someone sit the LSAT for you in person, probably in the low five figures. I've never been involved with people actually being paid for such things but they do happen. Huffman allegedly paid $15,000 to get her daughter unlimited time on the SAT. Paying to have someone actually stand in would have certainly cost more.