I picked up a single-strap backpack at a bargain store for $5 or $10 a few years ago. It's big enough to hold one or two camera bodies and about as many lenses.
The drawbacks, to be honest, are that it's not quite big enough for my liking, and I would kind of appreciate some padding built in. I added a little padding to cushion some of the shocks, but I suppose I got enough for the price I paid.
The really nice thing about this pack (apart from the Canadian flag stitched on it... ) is that you can swing it around to rest on your tummy in crowded subways etc. and when it's in the front position, the zippers are very handy for removing gear without taking the pack off. This one is pretty much worn out, so I imagine in the not to distant future, I'll have to invest another $10.