It really depends. I am having problems with mine right now. I gave a guy a jump start with my car running (not a good idea, apparently), and fried something..may be a fuseable link or voltage...
~ $1,000 on ebay, but many parts stores will test them for free. The easy way to tell if your alternator is bad is to drive the car. If it stalls while driving, the alternator is definitely at fault....