We're never accused of "bait and switch" because we stick to our price. If I bid $100 and it take 2 days, then we still only bill $100. On the other hand, if I bid $5000 and it takes 4 hours(won't happen,but), we still bill $5000.
You have to be a good estimator and look for potential problems and figure them in. Occasionally we include an "if-then" clause in the bid. If we have trouble, then you're going to pay more. But this is rare.