They're probably estimating that based on a worst-case scenario-needing inner rockers, rear pivot mounts, seat rail mounts, etc...take it from me, its really hard for a shop to estimate jobs without taking a really detailed look at what's to be done.

You've got to figure in the paint work that's necessary, the interior disassembly that will be required, etc.

With that said, it does sound a little high though, and some of the follow ups suggest some good alternatives.