Mr. smith built a rectangular-shaped sandbox for his children, measuring 4 feet by 4 feet by 6 inches. sand has a density of approximately 100 pounds per cubic foot and is sold in 50-pound bags for $3.50 each. how much will it cost mr. smith to completely fill the sandbox? mr. smith built a rectangular-shaped sandbox for his children, measuring 4 feet by 4 feet by 6 inches. sand has a density of approximately 100 pounds per cubic foot and is sold in 50-pound bags for $3.50 each. how much will it cost mr. smith to completely fill the sandbox?