I have an older Opcode Sonic Port box. Its a USB Stereo analog in/out to the PC and SPIDF digial I/O too. Small box with great sound. $25 plus $15 shipping.
you could also look for a used echo mia, which you might be able to get for $50. new, there is the behringer uca202, which is a 2 in/2 out (rca connections) usb interface for about $30, or the behringer fca202, a 2 in/2out (1/4" ts connections) firewire interface for about $80.