I think males are cheaper because they are replaced much more easily. Let's say you have a banana male and you breed him to some female and get a banana combo male. Are you going to keep using that original banana male? Probably not, because you have a combo now to work with. The original then goes onto the market. As more and more people ditch their less powerful males, their price goes down.

Theoretically, with every generation you can breed a better and better male, so you'd have no use for the male before it, as pairing the earlier male would be a poor choice of action when you have a better powerhouse. With females, you're not "losing" anything by breeding them. They are going to keep their value because the new powerhouse male can breed to them.

The only thing that can really negate this is that older males can breed more females than younger males.