Starting with Piecewise functions in terms of v = x*y
goldPremiumFunction = Piecewise[{
{400, 0 < v < 8000},
{0.05*v, 7999.99 < v < 25000},
{125, 24999.99 < v}}]
goldFunction = Piecewise[{
{90, 0 < v < 3000},
{0.03*v, 2999.99 < v < 24000},
{720, 23999.99 < v}}]
silverFunction = Piecewise[{
{6, 0 < v < 600},
{0.01*v, 599.99 < v < 37000},
{370, 36999.99 < v}}]
plot v up to 40,000
Plot[{goldPremiumFunction, goldFunction, silverFunction},
{v, 0, 40000}]
then inject x*y to replace v to show the x, y surfaces.
Plot3D[
With[{v = x*y}, {goldPremiumFunction, goldFunction, silverFunction}],
{x, 0, 200}, {y, 0, 200}]