Depends on the item. Some I break even when I consider the time I spent looking for the item, buying it, and selling (I calculate at $10/hour) and eBay/paypal/Andale fees. On many of them I make about 50-100% profit. Like the Nomad IICs, picked up 2 for $34 each, selling for over $50 each. It's all about luck in picking the right items. And never hold an inventory.
Yard sales are great too, as well as estate sales. I was playing around with antiques at one point but I gave that up since it's too time consuming.
Good months I've had profits of close to $1000, bad months maybe $20 or so when I've been busy or lazy and wasn't able to put up auctions. My main items are import games though, from Japan which I get when I go home.