You have two equations to consider:

V = πr^2h,

SA = 2πrh(side) + πr^2 (top) + πr^2(bottom)

thus: 500 = πr^2h, h = 500/(πr^2)

SA = 2πr(500/(πr^2) + πr^2 + πr^2

SA(including cost) = 0.4[2πr(500/(πr^2)] + 0.4[πr^2] + 0.2[πr^2]

I think now you find the derivative which should tell you what r is, and then plug in to find the dimensions.