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.
Related variants of this product are tracked as separate benchmark lines. Match the spec before comparing quotes.