Jon has articulated very nicely what I was feeling.
This is nearly as daft as saying 'Please tell me how to get Tescos to sell me bread at 69p when they charge 79p currently".
Builder want work (or not, depending on how busy they currently are, and whether they think the customer is going to be trouble) - they don't deliberably quote high, or they don't get enough work.
Building work pricing operates by competition - not solely by the fixed costs incurred by the builder (materials plus labour).
The way you maximise the competition is by getting three good builders to quote against the same specification. If you ask six builders to quote you are wasting the time of five of them and they won't thank you for it.
If you ask one builder to quote (and he knows it), then there's no competition - so you are likely to get a higher price quoted (though not guaranteed).
To some extent, you can trade quality, cost and time. Try and strive for two out of three of these, but don't expect a high quality job in double-quick time at low cost.