Cat, use the search feature to find ALEKS and EPGY... there've been several excellent discussions in the last few months.

As for EPGY, the answer is near the bottom of this page:
http://epgy.stanford.edu/applyandregister/tuition.html#info
In part:
"If a student completes a course mid-quarter, he or she may register for the next course in the sequence for no additional tuition charge (although registration and shipping fees will be charged whenever a student begins a new course)."

I recall, however, that there is a cut-off in terms of how close to the end of a quarter you can get and still be able to start a new course.

Here's a great page @ Hoagie's with some good info on EPGY. You'll also see links at the bottom with info on distance-learning in general:
http://www.hoagiesgifted.org/epgy.htm

Being offended is a natural consequence of leaving the house. - Fran Lebowitz