C.sterbai have not been available for very long, compared to many of the other corys. Since the oldest tank raised breeds, such as the bronze and albino C. aeneus and the peppered corys (C. paleatus) have been around a long time, and are commonly found, they are cheap.
Within the last year, I've seen sterbai here go for up to $25 for full grown ones, but now I'm finding smaller ones for only $5 or so. While these are probably tank raised, I recently bought some full grown wild caught ones for the same price.
It's just a matter of supply and demand, I guess. They are no longer considered a "status" fish,
While they are not too difficult to breed, they seem to me to be slower growing than bronze corys, so I suspect the price might stay at least a little bit higher.