yes that price is crazy...
I picked up my only GJ back for $50 a few years back on eBay, and it was EX+/NRMT if not slightly better.
But like Leon said they are one of the scarcer backs. Im guessin someone (or at least 2 bidders) just had to have it and didnt want to wait for a good deal (real value price) to come around...
they are worth a bit more than $50 now
, but no where near $600+ for a common player in lower grade