The Diecast K0 is the most sought after CB750, after the sandcast. A really good diecast K0 will sell for $10K plus, or about twice that of a good K1, (my favorite) or K2.
The K3 to K6 (while even better bikes, technically) will go for under $3K usually, unless they're "as new".
The F0 and F1 will go for less again, (great bikes, but not collectable) the F2/F3 is a hand grenade just waiting for a careless owner to pull the pin, so it falls into the F0/F1 price range. (cheap)
The K7/K8, while being the most "evolved" of all the CB750's, was seen as "too little, too late" by the market at the time, (whooped by DOHC Kawasaki's and Suzuki's sales-wise) and unless you're wearing rose colored glasses, a good one shouldn't fetch more than a couple of grand, tops.