I've done a lot of digging and it seems as though the prices vary quite a bit. As far as I can tell, Rich had 4 of them on Ibanezrules.com's used section priced between $1325 - $1600. There were also a couple on reverb priced between $899-$1350 but the 899 one has been modified and had holes drilled into it to fit a synth and advertised by numerous sellers over a period of 3 years at varying prices. It appears as though a fair price for an unmodded excellent condition js6 would be in the 1300-1600 range. If you drilled holes into it or sanded off the logo and replaced the pickups with Epiphones, you're looking at closer to 900-1000.
As with all things it appears to be highly dependent on quite a few facctors, but it does appear as though a js6 would fetch a bit more on the used market compared to a js1600.