I bought and installed a simple PCI one for the old man, and it was something like £2 off ebay. Works well
Equivalent in PC World is £20.
Bought a wireless modem router for same, off Ebay. £8 delivered. PC World Equivalent £40.
The only time you would pay more is to match a high end card with a high end router, like both Wireless N, if range/walls were a problem.
These things are really robust (if you feed your dog). **** PC World retail prices. I only go there to gen up on what I should be getting cheaper elsewhere.
For best results, also get one of these. They last long, very strong, and will save you a fortune in cabling and repairs
http://tinyurl.com/4bw3yh