As clients work to understand the nuances of manifesting their dream project, they are sometimes confused by actual costs of things. As a contractor I work to educate them on the realities of what the job entails. Oftentimes there is a difference between a "budget and an estimate." If you have a professional designer they are usually pretty well versed in what things cost. They can help you specify the scope of your project, so that when you get an estimate from a contractor you can compare apples to apples with different bids. We will sometimes charge a nominal fee to develop budgets, since this is often before any final bidding.
Estimates are costs after all the items have been specified by a professional and you have working drawings. Drawings allow the contractor to give a better estimate based on the "fit and finishes", which are costs based on the level of workmanship and the quality of the fixtures. If you want the Taj Mahal with gold finishes it's going to be more expensive than chrome with a standard finish. The contractor can often provide "an allowance" for some fixtures without knowing what the actual specifications are. These are placeholder estimates to allow for a final lump sum bid.