senghing's prices are pretty spot on to me. Of course, rarity of accessories and options also complicates things as well. A classic or Minimag with a complete splash ano kit will fetch much more than a stock looking one. Centerfeed body Minimags and Euro-feed Classic RT's also command more than regular versions of those guns because of the rarity of those body styles. Now there is also a plethora of different aftermarket rails and trigger frames that can also change the price from that of a 'normal' mag. Condition is also important, because if you find two Minimags in stock form, but one sat in a closet practically unused for 10+ years and the other was played hard with I would pay more for the unused one of course cause it will be in much better condition.
Often times, you will also find sellers online who have been out of the game for years and don't realize how low the resale is on paintball guns nowadays. They dropped $600 for a bone stock Minimag in '96 or over $1000 on a pimped out splash one and still expect to get most of that money back.. That ain't happening. Usually the market will correct them.
It's harder to get really good deals on respectable forums like MCB and Automag Org, because most guys know the market and pricing. It's a little easier on PBN were your dealing with under-age dummies who know very little. You can get some crazy deals on Craigslist if you look hard. You find people selling stuff they just want gone and don't care what they get for it. Thats also were you can find lots of nice condition 'closet guns' that just sat in storage forever and aren't banged up. You have to be carefull werever you shop online though, and you are taking a chance buying that way. I've never worried about dealing with people here or on AO but you never know.