If you want to use the stucture in the OP,

G
H
I
20
$ 750.00
$ 37.50
H20: =SUMPRODUCT((G20 > {0,500,750}) * (G20 - {0,500,750}) * ({3,5,7.5}% - {0,3,5}%)) + 10


The last term is the differential rate between tiers.