Presumably, the cost of taking the tests is the cost of having someone thoroughly vet your work, in addition to developmental costs. If it costs, say, $1,000, you'd expect that to mean that people would spend around 10 hours actually verifying your work. And so, even if you have to write 10 papers and develop 10 projects, 10 hours of grading should be plenty to provide a fairly comprehensive assessment (30 minutes per large assignment is much more than most professors put in now. At least, in my department). For $1,000, you could even up the evaluation time to something like 40 hours (at ~$15/hr, plus administrative costs) by using TAs who really understand.
This is why college won't get any cheaper: there will be more value in having a qualified person evaluate not only the test, but all your schoolwork. You can game a test, but it's harder to game a few years of school.
