Personally, avoid the cheap stuff.
I've seen countless parts that were "just as good as the more expensive ones" fail in the field on friend's guns and more so in the classes I give.
Especially when it comes to sling mounts and swivels... not many things worse than your rifle bouncing off the ground when your cheap eBay sling swivel gives it up... seen it.
I suppose a bi-pod mount won't be as critical... but you always run the chance of a crappy fit. I had one years ago that wasn't offset enough for the tightening mechanism to fully tighten.
You should be able to get a good brand name one, MagPul for example, for $20-$22 plus shipping. I need two myself and that's the one I decided to get.