Gas leak in my attic costing $275?

I would say so.

My first thought is WHY is there a gas line in the attic?

They are USUALLY run UNDER the house and in the walls.

Has there been a smell of gas in the house?

If NO, then there probably was NOT a leak, since gas has a chemical addition to make it smell bad, AND gas is heavier than air and would have settled down throughout the house.

Did he advise you of the leak, and get your authorization, or just fix it without asking?

THIS is NOT what he was there for, so to make a repair without your authorization is illegal in most places.

Is he also a licensed plumber, or just an AC technician?

In MOST places he has to be a licensed plumber to work on gas/water lines, and in some places gas requires a separate license.

I would want a properly itemized bill, among other things, before I paid for any repair with this gas repair separate from the AC service.

(I wish MY gas bill was only $37!!)

I'd get a second opinion.

Having said that, if you see how gas pipes are installed, they might have to do a fair amount of labor to fix a leak. The pipes are threaded together from the source to the destination, and you cannot just tighten a connection. The plumber will cut the pipe whose threads are leaking, thread the cut ends and install a coupling. That shouldn't typically take more than an hour or so. $275 for that sounds a little on the high side. (well, okay, a LOT on the high side - I'm thinking more like $100, unless access is really difficult and it's 150 degrees up there)

ME, we ll first of all a gas leak is a dangerous issue, im assuming the leak has been isolated ? if so, depending on the repair factors, sounds like your being charged for 2 stop valves and the rest seems to be labor, typicall plumbers are charging 75 to 100.00 / hour nowadays.... the valves should be no more than 15.00 ea. with markup - basically, your labor is 245.00 - if all is accessible, this might be a bit high - old brass lever type valves have a habit of leaking at the core. im assuming this is the leak since valves are being quoted in your paper bill. worst case, get a second quote -

You should always ask for an itemized bill and I would look into getting someone else to do my repairs next time. Anybody worth his salt would have showed you, called you, relayed the message to you somehow what the problem is, then ask if they want you them to go ahead and fix it or cap it off until you decide what your going to do. There are allot of crooks out there that take advantage of people and will double bill them. Don't get me wrong, there are allot more honest people but it only takes one to give the others a bad name :(