Cheap Mac Mini
Cheapest Mac Mini on Amazon
Cheap Mac Mini -- Straight From Apple!
The problem with getting a cheap Mac Mini is... well it's a Mac. And people love their Macs. They only get rid of them when they absolutely must. The Mac Mini is the most energy efficient desktop on the market, using less than 10 watts of electricity. The Mac Minis are appealing to buy because they don't have that horrible mid level tower. Made with aluminum and with easy access to memory, these are some nicely designed desktop computers.
One place you can go is straight to Apple and see if they have any refurbished models. The nice thing about getting one of these from Apple is that they tend to give you a warranty just as if you bought it brand new. These can be bought for around $500, which isn't too bad.
Doing a search on Craigslist you can find G4s for under $400, but I would go with the refurbish option unless you can find one for under $300. The reason is not just the price, you can get better than just the G4s from Apple.Go back to Apple's refurbish page often, they do actually update this page all the time. These are better than just basic sale prices. Like I said, $500 or even less for a protected Mac Mini.