Well, based on what I've seen on craigslist and eBay, you can look at about 250-300 for a used 8GB 3G, and 350-400 on a 16GB. Don't let anyone charge you more than 400 for a used 16GB. There's cheaper alternatives out there.
Here's what I suggest, if I were you:
Look around for a broken iPhone 3G. Make sure, and I mean make SURE there is NO water damage to it (this can be found in the headphone port and on the dock connector). If there is NO water damage, you can take a broken iPhone in and have it replaced with a new 3G for 200 bucks.
One of my friends gave me his broken 16GB 3G and I took in and paid 199 and got a brand new one. This is something to consider if you want an iPhone but don't want to get ripped off.
EBay seems to be a good option for getting a broken or used one for a pretty good price, check there.