Lucasi doesn't let u buy butt only. Im not sure how easy it would be to sell the shaft.
What do makers charge? I already have predator stuff and would like to keep everything interchangeable.
I don't think that anyone sells a butt without the shaft.
I've only seen Joss do that but they don't use uni-loc and don't have a classic SP.
For the Price of a Predator butt (if you can find one) you can get a full Lucasi cue with the shaft and some change.
BTW, the last Predator SP cues (classic look) had radial pin and nor Uni-Loc.
I don't know what custom cue makers charge, it's like asking how much does a car cost? different cue makers have different prices.
Best thing that you can do is ask the big on-line stores what they offer you, maybe they have an old predstor in stock, maybe they will sell a butt only, maybe they can make you one.