i posted a similar question a few days ago, after some research i came up with a couple of pretty good choices below $50
SB0770 | Creative Sound Blaster X-Fi Xtreme 7.1 SuperBiiz.com - Creative Labs Sound Blaster X-Fi Xtreme Audio 7.1 24-bit PCI Sound Card, Retail
a really good place to check for cheap computer parts is
http://www.pricewatch.com