Almost anything CAN be altered. Suit and tuxedo jackets are fairly complex pieces of clothing, though. The more complex, the more a seamstress or tailor will charge you to make it right. You're in a better position with it being too big than too small because it's easier to take away fabric than add it back in.
I wouldn't begin to guess how to estimate what it'll cost you, because there are so many variables to consider (if the jacket is cut such that altering is easy or hard, how much local people charge for their work, who you go to (skilled amateur, a professional tailor, the men's warehouse, or a dry cleaning shop, for example), how much has to be taken in and where, whether it's a rush job, etc).
I would recommend that you go to the yellow pages and find four local people who you might take it to. Ideally have him to take it to them and ask for an estimate (if he goes, they can see how much needs to be done). If he's not available, see if you can get an idea of how much it would need to be taken in, then give them a call and assure them you're only looking for a rough figure of how much it might cost.
In some places, alterers charge $10 an hour, some charge $30, and some just make a project price (like $50). People vary so much, the only helpful number you're going to get is from the people you might actually employ.