There is some truth to the saying "You get what you pay for"
This has been a frequently reoccuring topic I've discussed with people lately.
Yes, there are USB mixers that can be had for very little money (comparitively speaking). One of the things that you need to consider is that an audio interface was designed specifically to be an audio interface. A USB mixer is usually designed to be a mixer, and "Oh by the way, I guess we'll add a USB jack." This is mainly done so that companies that don't have anything to do with the home studio market, can get their toe through the door. Not a bad idea, and you can't argue with the affordable prices, but the audio quality probably isn't going to be there for you.
If you'd rather use a mixing console as an interface, then you will need to look at things like Presonus' StudioLive series, or something similar from
Yamaha, etc...
The StudioLive series of mixers comes with the same preamps as thier interfaces, and ships with Presonus' DAW,
Studio One. If you decide to go this route, be prepared to spend $1,000 and up to get yourself a nice board that can work as a high quality interface.
I'm not saying don't get a little Mackie board or
the Xenyx or whatever, but don't be too surprised if your buddy's $200 2 channel interface sounds better than your $200 USB mixer.