These devices are for sale at $200 per unit, and somewhat less in bulk. Make sure it's in working condition and not a scam, etc...
I'm looking into buying one myself, to have some fun. It needs some serious hacking, finding a proper way to supply power to it, some modded software, I'm not sure. Search "Gridseed" around the forum.
You will not get 500 Kh/s out of it, perhaps 350 Kh/s is more realistic.
Note that some people are dumping Gridseeds because KnC announced 250 Mh/s miner at $10k. When they release, you'll get minimal return out of Gridseeds. Edit: I don't vouch, even less for a pre-order, but check out this example group buy for the cost and the hashrate offered:
https://bitcointalk.org/index.php?topic=522707.0