There are plenty of options out there, but most of them are designed for audiophile purposes, so it's not as simple as a dongle, as there's a bit of processing involved...the cheapest (in all senses of the word) I've come across is the one offered by Yeo at diyparadise:
DIYparadise.com
he's also got quite the following for affordable amps, preamps and DACs too.
The USB -> DAC I'm looking at is made by twisted pear audio, twistedpearaudio.com...it'll be more like $300-400, as the USB module is outputting I2S into a fully dual mono DAC with Wolfson chips, along with the usual SPDIF inputs.
Hopefully that first one can help you out.
