First off WELCOME to the best board on the net!
Zoomin hit the nail right on the head. The first guy is probably only cutting out the rust and welding in the new pieces. I think the second quote is a little on the high side but he may factoring in the unexpected.
From the looks of your pics, you are going to have a lot of rust on both sides to deal with. Sometimes you can find really clean cabs or ones that have already been repaired for a good price on Ebay, here or on Craigslist. I picked one up local for a $100 and only had to cut out minimal rust. Just a thought, good luck with your project!
