Prices for sound cards range from inexpensive and up.
Here is one that is listed as Windows 7 compatible. I know nothing about it other than I found it on the www.newegg.com
site (where I buy a lot of my PC components). It is $24.99.
Newegg.com - Rosewill RC-702 7.1 Channels 16-bit 96KHz PCI Interface Sound Card
There are a lot of other PCI sound cards listed but many are not Windows 7 compatible.