At the show I went to recently ( a really small show though) the least expensive spider was $150 and I think that was only because he was newly hatched (so no meals in him yet) and quite small.
I will sell my normals at $20 for males and $40 for females. In my area the least expensive you'll find a ball python is about $40 at a small pet store for either gender. (not a big chain store) So I figure those are fair price.