Wifi cards are not expensive to manufacture. Wifi cards that work well and can handle a good amount of traffic at good speeds are a different story.
Personally, you'll need to look up the particular models you are looking for 'cheap' and see what they'll take. You'll be somewhat disappointed if you get a card that'll only handle 100Mb when a few $ more gets you 1000Mb. That's going to make a sizable difference to lag in games, downloads etc.
It's definitely a case of You get what you pay for.