V= pi(r^2)h and NOT 2pi(r^2)h as you have it.

The material for the top & bottom is: 2*(pi*r^2)

The material for the sides is: 2pi*r*h

Cost for top & bottom: 15*2(pi*r^2)

Cost for sides: 10*2pi*r*h

V = 20 cm cubed = 0.00002 metres cubed = pi*r^2*h

So: h = 0.00002/(pi*r^2)

Total cost = pi*r*(30r + 20h) = pi*r*(30r + 20*(0.00002/(pi*r^2)))

I assume you can take it from there.