Hoontech makes add-on daughterboards for the ubiquitous and cheap Soundblaster LIVE series of cards. It's what I use in my computer - SB Live to CI Audio VDA1 DAC via toslink. Soundcard + daughter board should set you back no more than $70 total. It's not a Cadillac solution, but I'm very happy with it. It adds both toslink + coaxial ins and outs.
