I've used one of the $20 eBay bags before with a point-and-shoot in a pool and had it work just fine. I got the bag for something to play around with, to use with a small Canon Powershot P&S model. I used to lifeguard at our town pool in high school and college, so I got it just for shooting underwater with my coworkers - nothing serious.
The main problem I had with the bag was that the circular black rubber area for the lens to go would shift and vignette on the lens. You had to be careful to center the lens on the port, otherwise you'd get black around the edges.
I'm a serious photographer who uses DSLRs and expensive lenses on a regular basis (topside, that is). So a $20 bag for my $150 point-and-shoot was fine with me; I wasn't particularly worried about breaking or losing the camera (I would have been bummed, but it wouldn't be like destroying my ONLY camera) and the loss would have been relatively minimal. If your P&S is your only camera, it might be a different situation for you.
If you DO go the bag approach, be sure to test your bag first by putting some paper towel in it, sealing it, and submerging it in a bucket of water. Squeeze the bag a bit and play around with it as if you were shooting. This isn't a foolproof test obviously, as you won't be simulating the pressure of being several feet below the surface, but it will at least give you some reassurance before putting your camera inside. Also, don't go deep with the bag at all. Its a good idea to know how deep you can dive and get yourself used to what pressure feels like underwater so you can be careful not to dive too deep with the bag. To be on the safe side with the bags, I'd only go about 60%-70% of the maximum depth rating.