I paid $600 for a never used set with K&N's. New they are $800 plus filters. The filters are expensive and you have to pay attention to which manifolds they come with. They just screw in and if you need new ones they may be expensive. Also a set jetted for another bike might mean you have to pay another $100 or so for jets, needles, slides etc,...
I'm guessing that GPZ set would need another $150-200 spent on manifolds and jetting to work on a CB??
The nice thing is that they have adjustable spacing to fit any bike unlike the mikuni smoothbores.
I've been looking for the older style CR's but I hear they don't have idle circuits??
The cheaper option would be old CB750 carbs or if you want to make new manifolds, some 28mm mikunis off a Z1 or GS1000, I ran these on my old racer. Expect to spend lots of time jetting for such a different application.