If you can put up with Dolby Digital for most things, and DTS for films that support it, I would recommend a second hand HDA (old name for Auzentech) X-Mystique. That card's based on the same chipset as my Club3d Theatron DD, the C-media CMI8768+. These cards encode everything to Dolby Digital Live on the fly and using PowerDVD or similar will pass through DTS encoded signals to a receiver. The X-Mystique has a digital coaxial out but has been discontinued. I saw one go for within your price range on a well known auction site last week.
Unfortunately the Club3d card only has optical digital out and they're just under £30 new. There are definitely a few cards based on the CMI8768+ chipset around (note the '+' the standard chipset doesn't do Dolby Digital Live). If you don't care at all about analogue quality look for one made by Asonic. It's optical not coaxial but they can be found for a little over a tenner. Add on an optical to coaxial digital converter for £10-15 and you're in business.
Edit: another card with DD and coaxial out - Trust 714DX 7.1 (AKA SC-7200).
Further Edit: and another - Trust 514DX (AKA SC-5250)