When did you last check? A full horned rhino is definitely in more demand, especially it seems when 22" or more.
For a few years there was an option of leaving the real horn behind for a significant discount. They would keep ownership with the farmer who owned the rhino and he was basically speculating that the horn trade would open and he'd be able to sell it. Last I've heard most are no longer even wanting it.
I would encourage hunting a very mature old bull that had been dehorned. Especially if it has grown back significantly. You can't replicate the big base those bulls have but you can replicate the pointy end
Edit; you can replicate the entire horn.. but you cannot honestly put a horn with a much larger base onto a skull that is young or had a much smaller base.
I didn't get a picture, but there was a display of rhino shoulder mounts in the hallway at sci. One had a particularly large base on his horn. It was very noticeable how much difference there is