Hmmm....for a '75? For a fully restored K0 or certainly a Sandcast, $15,000 would definitely be doable (Sandcasts could probably bring upwards of $25,000). Don't know about a later one being worth that much, though I'm certainly not any kind of expert in the CB750 realm. But I'm pretty sure that the earlier ones would be more desirable to a collector than a '75, though a '75 is certainly a nice bike.
Of course, to even get to that level of value, that bike would have to be perfectly restored -- proper bolts, screws, plating, grips, down to the last detail. Plus, perfect paint and everything else that goes along with it. These VJMC guys go crazy over this stuff.
I don't think that a cafe'd 750 for $1500 is unreasonable -- as I mentioned earlier, maybe you might just need to find another outlet to sell it that would attract that kind of buyer. Let's see a photo of it!