How much should it cost to replace an alternator?

I have a corsa 2005. Last night all electrics died and today the AA recovery man took it to the garage and told them to change the alternator and check my battery. I did Not get a quote, being so far from home all I wanted to do was get back. They should start the work tomorrow. I really hope I don't get ripped off. Can anyone give me a ruff idea of what I'd expect to pay in the U.K