Well, if you get a female yellow belly you can make Ivorys. YB pastels are nice because the YB gene cleans up the pastel gene and makes it brighter. Those are probably the two most affordable morphs. Then there are Lessers and Mojave for slightly more. The combos between them and a YB are not as dramatic but still nice.

Go to world of ball pythons and start looking up different ball pythons that carry the YB gene. You can get an idea if what the different combos look like.