GGB Through a bill of quantities. A BOQ breaks the fit-out into measured line items — quantities, specifications and rates for every trade — so every contractor prices the same scope and the bids can be compared line by line. Without one, the lowest quote is usually the one that measured least, and the gap comes back later as variations. Freeze the design before tender, have every bid priced against the same BOQ, and award on comparable bids only.
From Choosing Your Restaurant Architect, Kitchen Consultant and Contractor — the full reading, with the method behind this answer.