I'd agree that it's a good price, if it has the JP shield inlays and the matching headstock... I probably wouldn't go any higher in price if it didn't have the piezo like you said.
I bought a beat up fully loaded Mystic dream JP6 that had been resprayed (that's another story) for $600.
So a mint JP6 even without Piezo for a "bag of sand" + $100 seems ok to me.