Depends on the weight of the snake, if she's a proven breeder, etc. If she's a big snake and she's laid good sized clutches, I'd say she's worth it.

Babies are less expensive because there's been no work put into them. A big, beautiful, proven breeder has had a lot of the hard work already taken care - she's been grown and fed and you know she's viable and immediately ready to put into a project.