I'm not aware of any 1) low cost, 2) high quality, 3) external, 4) multichannel interfaces. If you're willing to forgo one of those four you can find something pretty easily.
For example, there are some fairly cheap high quality external 2 channel soundcards/DACs floating around. Some of the Fiio series DACs can be had for a little more than $50 (e.g. the e7k or the e10). I've had an e7 for over a year and like it quite a lot, although it's bigger somewhat more expensive brother (the e17) is better and has ASIO drivers. See this thread for info on the e17:
http://yabb.jriver.com/interact/index.php?topic=78637.0However, it's a very different world in multi-channel territory. You can get pretty solid multichannel internal PCI-E cards (like the Asus DX) near your price range, but it's very hard to get the same quality in external multichannel interfaces at a low cost.
Check out this thread for more info on what's out there :
http://yabb.jriver.com/interact/index.php?topic=55607.0, but a lot of the good options are in the multiple hundreds (or thousands) of dollars. I can personally recommend the Steinberg UR-824, but it is not cheap.