well, yes, that is true..... you don't always get your money's worth out there. and of course, there are also some awesome deals out there too....
but i still believe that you can get better performance simply by just buying a higher quality audio interface, rather than taking the digital out of the Sonica, which is a low-end interface to begin with.
but unfortunately, i was mistaken... i couldn't find too many interfaces out there that use USB/firewire in his price range. so, sorry... though i think if you're really patient, you can find some good ones on ebay selling used that would be in the $200 price range. but i think the cheapest model that's high quality would be Digidesign's Firewire interface, and unfortunately, it's Mac only.
personally, i have a MOTU 896, and it uses firewire, but of course, it's much too extravangant for this type of application.