By board foot (1 sq ft × 1 inch thick). Closed-cell: $1.50-$2.50/bf. Open-cell: $0.50-$1.20/bf. A 1,500 sq ft attic at 6" closed-cell = ~$8K; at 6" open-cell = ~$3K-$4K.
More detail
Board-foot pricing accounts for both surface area and depth. A 1,500 sqft attic ceiling sprayed at 4-inch depth = 1,500 × 4 = 6,000 board feet. At $1.85/bf average closed-cell pricing, that is $11,100; at $0.85/bf average open-cell pricing, that is $5,100. Most Cincinnati attic installs spray 4-7 inches depending on R-value target and rafter depth. Wall-cavity foam in standard 2x4 walls fills 3.5 inches of depth × wall surface area. Rim-joist foam at 2-3 inches of closed-cell on perimeter band-joist surface area. Quote breakdowns from credentialed Cincinnati installers list the board-foot total per area and the per-bf rate, so the homeowner can verify the math. Watch for quotes that omit thickness (a cheap-looking bid at $1.20/bf for closed-cell may be quoting only 2 inches versus a competitor's 4-inch spec). Cincinnati quote-comparison guidance: when comparing quotes from multiple installers, ask each for board-foot total per area and per-bf rate explicitly. Some quotes hide thickness assumptions in summary numbers; a $1.20/bf closed-cell quote for 2 inches of depth is a much different scope than a $1.80/bf quote for 4 inches. Total project cost is what matters; per-bf rates are a comparison tool but only when normalized to the same depth.