I designed a job once and one of the bidders was 25% of the other lowest bid. The overall job was in the 150-175k range. I called him, told him he was a bit low and wanted to know if he wanted to resubmit before the deadline. I couldn't tell him the lowest number, but I told him he was so low that I was afraid he was going to lose his butt. He added 10k to the job and he said he thought he was fine, he got the job, did not realize the complexity of the submittal process and he lost his butt. Signs turned out great, but I felt bad that he lost a lot of money on it. He was grateful that at least he added the 10k but probably should have added 40 more.
Those larger ADA sign jobs can be scary, out of whack bids scare the crap out me because there are always hassles if the bid was way off the budget estimates.