hmm yes.. wouldn't be a thrilling experience. Check out what they've used to create their preliminary site costs proposal. this might help gauge how accurate the costs are that you're looking at. I know when I was in the Sales process, it was purely a guess as to the typical conditions of that area, but my soil report which was presented to me at site costs/Tender appointment, although it was preliminary, it did show a lot of rock so we were able to factor this in early on and the costs didn't change.
Good luck