Why do normal Balls cost less than normal Boas but morph Balls cost more than their Boa morph counterparts?

For example (based on what I have seen in various spots including web sites and conventions):

Normal Ball - Avg $15-20 for baby
Normal Boa - Avg $40-50 for baby

Albino Ball - Avg $350-400 for baby
Albino Boa - Avg $225-275 for baby

Just curious.