Prices have been going up, like on all Colt revolvers, and a lot of sellers are talking them up like they're something rare and special. They might be relatively rare but they're not too special and they're certainly no Python, poor or otherwise, more the poor man's Trooper. I've never seen or heard of any major mechanical issues with them but the parkerized finish is rough, there will be tool marks, etc because that was where they had to scrimp to get the guns built and out to market. The 6" guns seem to be more common than the 4". I've wanted a 4" since they came out but have yet to come across the right deal at the right time.
Considering that I paid $450 locally for a 96% nickeled 4" Trooper MkIII and <$250 for a parkerized Agent from the same era a couple of years ago. And, that the uniqueness of the gun is due to the finish and the story that goes with it, it really doesn't have any other redeeming qualities that make it stand out, I'd be looking to give around $500-$550 max. Naturally local markets differ and how bad you want it is always a factor. For me, if I found a 98% or better 4" Peacekeeper I'd go to $650 and maybe a bit more, but that just because I've wanted one forever.