im going to disagree with this one.
ive been the low bidder many times... usually because the other guys "smelled" the customer's money, and gave extremely high bids for no other reason.
and ive been the high bid lots of times too, and got the job. last fall, i got a lead from an insurance agent. I was the 4th roofer to look at the house. when i got there, he said to me "well, you are the 4th, and last bid, and im making a decision today". When I climbed out of his attic, he said "you are the only guy who wanted to get up on the roof, AND in my attic" and when i wrote my bid for a re-deck because he had 2 layers of shingles on top of wood shakes, and i handed him a $22,000 bid for his 35 sq roof, he looked at me funny, and asked why mine was more than 2x higher than everyone else. i asked him to see the other bids, and quickly pointed out that the other bids were for either tearing off 1 layer of asphalt or two. nobody else looked past the layer of felt that someone put over the shakes before putting shingles on.
he felt that because i was the only roofer that talked about re-flashing EVERYTHING, and was the only one to even notice the shakes, he hired me on the spot. i ended up doing the neighbors house too.
sorry for the long story, but my point is not to throw anyones bid out just because of a number. use the best roofer for the job. check references, look at previous jobs they have done, ask about insurance and warranties, etc etc. there are LOTS of factors to consider besides price. dont let a couple hundred (or even thousand) dollars be the reason why you make a huge investment on your most valuable possession.