If it helps, this is exactly what our school is built around. Every class we create is available both in a degree track as well as ala-carte to students exactly like yourself. You have the same professor and t/a support, grading and evaluation. You also have the same access to the resources in our learning system. We are tuition only (no fees) so we even charge the same amount as the degree seeking students ($100/credit, $400-$500 per class, $5k for masters). And, if you decide to go through admissions and seek a degree, any courses you take would apply towards that degree.

The only real difference between in degree and ala-carte is that degree seeking students will have more options for how to pay for their education. Once we are accredited, a vast array of financing becomes available for students pursuing a degree. Some are good (grants, scholarships, state subsidies) some are not (Student loans), but having options for our students helps a great deal. We structured our program this way because we come from your exact position: why can't I take the classes I want without all the BS?

I think the reason more schools don't do this is that it would expose the true cost of these classes, which at institutions like Stanford or MIT can easily exceed $5,000 per class. Actually, after fees, housing and books, $10k is closer to accurate. The market for $5k classes is not huge, and without putting everything on a gigantic student loan credit card, few rational consumers would pay the price.