Having said that, there are a number of ways to "level the field" so that all the builders are quoting on the same thing.
The first one is to have a comprehensive written Scope of Works. The builder must know exactly what he is being expected to do. Just giving him a drawing will leave things open to interpretation and the builder will usually "cover" himself for items on the drawing that are not fully detailed or explained. If those concerns do not materialize, then he pockets that contingency, if they do, then he is covered. Either way, you pay.
The next point to consider is a specification.
(See "Your specification, do you know what your builder is building?" on the website.) The issue here is that without a specification, builders would not be quoting on the same things, right from the quality of the bricks to the windows (particularly aluminium) or paint to door locks and floor finishes. Some builders might be going the really cheap route while others may be quoting on a much higher quality product.
The last one is to get tenderers to itemize their tenders so that you can make direct comparisons on demolition, foundations, brickwork, plastering, roof, windows and doors, plumbing, electrical, as well as mark-up, etc. This will also give you the opportunity to see where one builder has a big mark-up on any particular item which you can then negotiate relative to other builders prices. It is also a good policy to separate items like purchase of tiles from laying of tiles, for example where you can compare labour rates of the different tenderers.
The last one relates to the builders themselves where contractors just at the moment are very hungry and some would be quite prepared to cut their profit margins (mark-up) just to keep their staff employed.
If you were able to present projects in the way that I have mentioned, then I am sure that you would get a better understanding of what each builder is quoting on.
The last option which I often advise as being the first option would be to employ a Quantity Surveyor, even on a relatively small project because the information that will be provided will tighten the tendering process up so much that you will probably spend less on the project, including the QS fees than you would have spent without that service.
I hope that this helps. If you need any further information, please contact me again.