A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
I'd use a lookup table listing the thresholds, the rates and the difference between the tiers. In the screenshot below, that table is in D2:F4 but it can be anywhere: on the same sheet, or on another sheet, even a hidden one.
The formulas look like this:
The formula in B2 is
=SUM((A2>$D$2:$D$4)*(A2-$D$2:$D$4)*$F$2:$F$4)
This can be filled down.