Tiered Pricing Commission Calculation

Hi Guys.

I have seen a lot of Questions/Answers on doing a Tiered Pricing Calculation, and now I have my own questions on this problem.

Scenario:

Between 1 - 500,000 Hits $1.00
Between 500,000 and 1Mill Hits $0.50
Over 1Million Hits $0.25

good news is here is the Formula I am using.... and It Works!

=SUMPRODUCT(--(C8>={0,500000,1000000}),C8-{0,500000,1000000},{1,-0.5,-0.25})

BUT,

Instead of hardcoding in the pricing into the array, how can I reference cells on the sheet, so that other people can alter the rates without changing the formulas.

I tried substituting the 1, -0.5, and -0.25 with A1, B1, C1... but all I get is errors.

Is there a certain syntax or something I'm not getting ???
or can an array not have a nested variable ?

Thank You