The alternator should be fairly cheap as far as labor goes but i do not know about the part. For the oil leak, no one can tell you anything accurate because the labor and parts cost all depend on where the leak is coming from. Please post more information if you know where the leak is coming from.
Question:: Is there oil leakin gonto your alternator??? THis will burn it out. Go to power wash and clean the etire engine bay. Watch for signs of oil leaking onto the alternator. Stop the leak, fix the alternator.