Priced per unit with quantity breaks. Vendors quote steps at 1 / 5 / 10 / 25 / 50 / 100 / 250 / 500 / 1000 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.
The same product family is tracked in 20mil stock. Quotes are NOT interchangeable across variants, so match the spec before comparing.