Answer:
1.78 feet.
Step-by-step explanation:
If they cost $20 each and its $2 per square foot, the area of the spheres
is 20 / 2 = 10 ft^2.
Area of sphere = 4 π r^2 = 10  (where r = the radius)
---> r^2 = 10 / 4Ï€
      =  0.796
---> r = 0.892 ft
So the diameter = 2 * r Â
              =  1.78 feet to nearest hundredth.  Â