First Tech on Hennepin takes trade-ins so they always have used ones to get rid of. I don't see anything really cheap on their used page right now but the iMac I'm using right now I bought in the store for about $300.
And Mac(!)phisto, deep down you know why you're trolling here.
I have an iMac I have been thinking about selling. WE bought a few years ago for my wife to use and it has had almost no use. PM me if your still looking and want more details.