For one square foot of spray foam insulation cost, it will cost you. There will be different costs depending on the professionals you hire to do the installations for you but the average cost for one square foot of spray insulation will be $1-$1.20 for open-cell insulation and $1.20-$1.50 for closed-cell insulation. However, you need to know that these costs are for a thickness of one inch. If you want two inches of width the cost will double and if you want three inches the cost will triple.