I asked for trouble and I got it.

I would definitely want to see the rusted lines with my own eyes. But, like you say, if AutoZone wants $600 then the shop cost isn't that far out of line, and I imagine you have to drop the tank to replace the fuel pump, so all in all it doesn't sound too outrageous. A bit high perhaps, but since I've never done it I don't know how difficult an undertaking it really is.

You could always get a second opinion. OTOH if you've had work done in the past and been happy with the work and the price, then you know that the work will be done correctly. And sometimes that peace of mind is worth a few bucks.