Unless I've made a gross error in calculating a bid, the price given is not up for negotiation. In my opinion, you lose credibility by lowering your price after submitting a bid. As for following up lost bids, I don't generally bother. Everyone has a different style of workmanship or what materials were used, so a lot of times it's like comparing apples to oranges. Also, there are some corners that I won't cut just to get a job.
