Cheapest grad schools per credit

[Ideas]

Looking at the site it says I can register with the intention of no credits, undergraduate credits, or graduate credits.
My thoughts about taking the class are for nothing more than personal enjoyment. My degree paths are more towards a business degree or a psychology IO degree.
Say I sign up for the option of graduate credits. What advantage would these classes have in the future? Other than for personal enjoyment/enrichment?

You could use them toward a certificate.

Grad credits can sometimes be used in an undergrad degree.

If you have an unrelated Masters and at least 18 English credits, you can apply to be an adjunct teacher of English at a community college. (However, 18 credits is the minimum, and they prefer someone with a Masters in English, teaching experience, etc.)