The purpose of both the tests(AP test and SAT subject test) is different. AP test is designed by faculty to test out good kids from average kids. The credits gets transferred when you get into college depending on your grade in AP test. SAT subject test would not transfer your credits even when you get good score in it. Although colleges do look at SAT subject test scores to see how good you are. Credits are transferred meaning you do not require to take that course in college and so you do not need to pay for it since it is considered that your knowlege is as good as taking a class in college.