The Costs. Minimum Cost: Approximately $.50 per square foot. Maximum Cost: Approximately $2 per square foot. The size of roofs vary immensely from a standard, urban roof measuring about 800-square-feet to suburban roofs, which often range above 1,500-square-feet.aximum Cost: Approximately $2 per square foot. The size of roofs vary immensely from a standard, urban roof measuring about 800-square-feet to suburban roofs, which often range above 1,500-square-feet.