When people get multiple bids, the deciding factor is the lowest price many times. This is a problem more times than not. The higher bid may not be off base, as the price may be reflective of the product the contractor wants to provide. The lower price may be your slam bam job. The client needs to compare apples to apples. Sometimes you want to toss out the high bid and the low.
Agree that the bid price is only part of the decision. I know I passed the lowest two bids on my heating system work simply due to the reputation of the company I finally selected and the fact that they were very responsive and seemed like they really understood what I wanted done and weren't going to cut corners to get there.