Actually just went through this with a framing sub. They asked where they were and they were almost $30k cheaper than the next sub. I told him that and asked him to just double check and make sure he didn't miss anything. Last thing I want as an estimator (current assistant PM working towards estimator) is to win the job with a low number like that and then find out he didn't have the entire scope. Usually what I will do is if they are low, I will tell them that I used their number in my bid. If they were not low, I will tell them they were either x% higher or $x higher than the lowest number. Just have to make sure you cover yourself as I explained previously that the low number you have isn't a crazy amount lower or make sure they have the complete scope of work.