That's the difference in cost between registering noncredit and registering for credit at the undergraduate or at the graduate level. You may wish to register as an audit student if you do not need a grade for the course but need to have your participation recorded on an official college transcript. Course requirements are established individually with the instructor.
Full tuition is charged.
If you are taking a course strictly out of personal interest, you may wish to register to take a course noncredit. In this case, no grades are assigned and no official record of your participation in the course is maintained. Tuition for courses taken as a noncredit student is reduced. The decision on whether to accept courses taken here as a non-matriculated student is made by the degree program to which you apply. If you're planning to apply for a degree program here at ESF, make sure you choose any courses you take with the advice of a faculty curriculum advisor in that program.