Up to 15 sq ft
Priced per unit with quantity breaks. Vendors quote steps at 1 / 5 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 24×18. Quotes are NOT interchangeable across variants, so match the spec before comparing.