Cost on ebay seems to range from $15 to 25 per card.

How often depends on a mix of luck, and how hard you push the fans, running at 80-100% can cut the lifetime considerably.
Some will fail within the first year or so, others will last 2-3 years, some may be running just fine +5 years down the road.

My remaining 7950/280x are nearly all on their second set of fans (a couple sapphires are on their third set).