I saw YB males going for $50 at the last show I went to. Granted they were not exceptional looking snakes but they were definitely yellowbellies. If I were ever to get a YB I'd want to get one of the more visually impressive ones, they do make great combos and Ivories are one of my favorite white snakes.
I'd say the "most affordable" visual morph right now would have to be pastels but prices on spiders and other great looking morphs are definitely coming down big time I've seen spiders for $150-200.
Sorry, somewhat off topic but if you go back a few years on Fauna Classifieds when spiders first hit the market they were selling for 20k each!