I've seen them come up for sale on this site about once every year, or so. They don't generally sell for much. The ones I've seen here sold in the $600-$800 range. That's it. Unlike the P7's and the P9's, VP 70's don't seem to have much commercial value. The only market for them at all is HK collectors who want one to round out their collections. With a 15 pound trigger, they are practically useless as a range gun, and are really just a cool historical piece to have.
I, personally, am into cool historical pieces though... which is why I have Steyr GB's, a Luger, SIG P230 and other weird guns to round out my collection of more practical and useable firearms like P7's, P9's, GLOCK's, SIG's, P2000, etc.
I've seen some people try and sell VP 70s at high prices, as if they were a pristine P7 with box and papers. I don't think that works out too well though. The market is pretty soft for VP70s because, as I say, people just try them out with a box of ammo and then they sit in the safe until the person decides to sell, as these guns don't exactly scream "take me to the range again" in the minds of their owers. I think this is a pretty well established pattern.
The best way to ascertain value is to start high, and drop the price $100/week until it sells. If somebody thinks the price is right, they'll let you know by giving you your asking price. Everything else is just talk.