They sell for more because they have proven to breed without an issue. I don't know the exact statistics, but dud breeders do happen. It's pretty much extra money for the guarantee that the snake you buy will give you eggs, instead of taking a chance like most of us do that plan to breed.

IMO (I have not bred yet so don't take my word for it) it's sometimes as simple of a fix as throwing in a different male, which can be frustrating if you have a specific combo you want. But, I have heard stories of no matter what, they just won't do it.