Price is what someone is willing to pay. If you can't find verifiable proof of one that's sold, what you paid is what it's worth...to you. Once you put it up for sale or auction you'll know if you over paid or not depending on how quickly it does or doesn't sell.
That said, GP games seem to be all over the map. Most are fairly low production with exception of the Sharpshooter series but that doesn't mean they all command high prices. Lock Ness is pricey. Cyclopes is getting there. Andromeda is not too far behind. I'd say Global Warfare would command a pretty solid price just due to rarity. Not sure what that would be though.