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}]