I spend my life quoting jobs, up to a million quid, so after spending weeks putting a price together it's a bit of a nuisance when you don't get it.
HOWEVER, there are some fairly good rules you should follow when getting a quote.
1. Make sure that all are quoting like for like - It's not only fair on everyone but also means you will get the job you expect in the end.
2. Try to get quotes itemised if possible - While getting someone to do you an itemised quote and then giving it to someone else to do cheaper is morally the same as stealing sweets from a child and blaming it on another, you should make a point of comparing individually generated itemised quotes before placing an order.
3. If they all come out with the same job but for three close but differing prices, pick the middle one! The cheapest one is generally too good to be true and the most expensive one is often not best value. The likelihood is that they're not ripping you off, just using higher spec materials or doing more remedial works to get the whole job looking perfect. You may not need this level of quality.
4. Don't be afraid to ask your builder to walk you through the project & quote. That way anything missed will be picked up and costed, rather than coming up as an extra over which everyone falls out. In this respect a one line quote is a recipe for catastrophic disaster. And you only have yourself to blame. Don't accept one line quotes!
5. Know your budget before you quote - If you have only 10 grand to spend, don't give everyone £10k to spend, but if quotes come in at £12k ask what could be omitted or down specced to fit your budget. And make sure any specific exclusions are noted in writing. That way anything which could be reasonably expected should be included. If they don't do plastering, it should say in writing 'plastering excluded'.
6. Ask for references - If you don't have any sound recommendations to work off, ask the builder for half a dozen references. And get off your behind and go visit them. You should talk to people they have worked for before, inspect workmanship, ask about working hours, punctuality, ability to stick to programme, site cleanliness, any propensity for extras, how they wanted paying, if they asked for materials in good time. All these things are ultimately far more important that the bottom line.
7. Just make sure you trust your builder - It's all very well him being a top chap but he may stick some hopeless ruffians on site to wreck your house and drink your tea. If he doesn't come through with good labour, haul him up on it. Make sure you set down some rules before you start and that he understands them.
8. If you must haggle, make sure you're basing it on sound evidence. This is why itemised quotes are the best way forward. If someone is charging £3k for something others are charging £1k, get him to justify it. Don't just ask for a discount. It never works out in the long run.
Hope this helps!
Simon