Sally sharpens lawn equipment blades in her spare time for extra income. Buyers of her service are willing to pay $2.50 per blade for as many blades as Sally is willing to sharpen. On a particular day, she is willing to sharpen the first blade for $1.75, the second blade for $2.25, the third blade for $2.75, and the fourth blade for $3.25. Assume Sally is rational in deciding how many blades to sharpen. Her producer surplus is.