To answer the question in your subject line, "Whoever makes the cheapest one that will do what you want, because they all pass Dolby Digital through the SPDIF exactly the same no matter how expensive they are." I went to Newegg.com and listed all sound cards by lowest price. I went up one by one, skipping over the Creative because I hate them, and the cheapest one with SPDIF is the DIAMOND XtremeSound XS71 7.1 Channels 24-bit 96KHz PCI Interface Sound Card
at $29.99 plus shipping. It has coaxial connectors.
If you want to jack your line outs into an older receiver that doesn't decode digtal, just for stereo music, I'd still go with this one, because I know it will work, and the price is already pretty low. You might want to do more with it once you get it, too. Just be warned that you will not get surround sound from games with this card through the SPDIF.
If you do get it, don't even unwrap the installation disc, you might get rotten driver juice on you. Go to C-Media
and download the driver for the CMI8768, or go to Google Code
if you want to try something different.