Originally offered in .221 "fireball", the hollow Zytel stock had chambers for added weights to adjust balance.
Back in '77 they retailed for $170, dropped in the mid '90s.
It was a single shot and there were a few models offered in other calibers.
Make sure the one you are looking at was a .223, not a rechambered .221, whoever may have done the rechambering or how good it turned out, who knows?
Prices start at $325 in good condition for the "varmint special" model all the way up to $750-$850 for excellent condition XP-100 custom HB long range pistols with a 10 1/2 inch bbl. or XP-100R KS with a 14 1/2 in bbl.
I see them at shows from time to time as there were a considerable quantity manufactured.
If it fits your niche, why not own it?