Pretty cheap for a really low end one ($20), to really expensive for the super-duper ones ($200+).
For your basic purposes, though, probably about $50-$100. Make sure you get one that you know will work with your computer. Worst thing to do would be to buy one, and it not work with your hardware setup.
I don't have any specific recommendations for where to buy one, as I never have before. Besides a local PC shop, eBay is the first thing that comes to mind. Once again, make sure you are getting one that you know will work (preferably the same brand/model, if possible).
Best of luck to you.