The area of a 200 ft by 300 ft area is 200*300 = 60000 ft2. To find out how many 500 ft2 rectangles it takes to cover this area, 60000/500 = 120. If 120 pounds of grass seed are needed, and each costs $3.25, then 120*3.25 = $390 is the cost of the seed needed to cover the whole area.