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.