Issue one scope document, one finish schedule, and one set of drawings to every bidder. Require unit prices for likely changes. Ask for exclusions and assumptions in writing. When inputs match, prices become comparable, negotiations become focused, and you can select for total value, not illusion.
Open with interests, not positions. Share constraints, priorities, and timing. Explore options like material substitutions, shared logistics, or schedule tweaks that lower cost without hurting performance. Fairness invites creativity, and collaborative bargaining frequently reveals unexpected savings hiding in coordination, waste reduction, or simply clearer sequencing.