Replacing a head gasket is a big job. I'd ask exactly what diagnostics were done to determine the gasket is blown before spending that kind of $$$. Let us know what they say so we can run it through the BS meter. Not saying it isn't posible, but you want to be sure before that kind of cash layout. We've got some real experts that can confirm if the proper diagnostics were done (and I'm not one of them).
Tell them you're going to have to ask your father/brother/husband (or some other man) about it before you authorize the work. I know that sucks (and sounds sexist), but a lot of shops will blow smoke to someone they feel doesn't really understand cars (especially women). Not saying anything about you (really, I'm trying to help), but doing that can't hurt. If their story changes, get the car out of there ASAP.
Hope this helps and good luck!
