Priced per unit with quantity breaks. Vendors quote steps at 1 / 5 / 10 / 25 / 50 / 100 / 250 / 500 / 1000 / 2500 / 5000 / 10000 units, and the per-unit rate drops at each break.
Usually a specification or finishing difference, not the core work. Ask both vendors to quote the identical spec and the gap normally narrows.