Hello fatboyle,
It all depends on where you are purchasing it from. According to EveryMac.com a 30 GB iPod has an estimated price range of anywhere from $90 - $150.
http://www.everymac.com/systems/apple/consumerelectronics/stats/ipod5thgen.html
So I would say anything thing in that range is game. If you purchase one from eBay, just be careful and make sure that you are buying a fully functional iPod. I would recommend looking around Amazon for good deals, and if at all possible look for refurbished units, which should be of the same quality as purchasing a brand new one.
Here is one page from Amazon for the 30 GB iPod.
http://www.amazon.com/Apple-Video-Player-Black-Generation/dp/B000EPNDEG/ref=sr12?ie=UTF8&s=electronics&qid=1285117611&sr=8-2
Hope this helps a little. 😉
B-rock