Because this is the easier of the two, we'll go into it first. The cost of a new alternator varies by vehicle make. Average is about $100 or so for 4 cylinder engines and $150 for 6 or 8 cylinders. Most mechanics can install a new alternator in about half an hour and most home DIY mechanics can do it themselves. A rebuilt alternator can be purchased for about half the price of new, but may not have the same warranty as a new unit. The cost savings often makes it worth the risk, however, and most will function as if new.