Hi

At first, a couple of remarks.

1. Programming in Excel is writing VBA code, p.e. some your own function. What you need, is an Excel formula, which is using Excel built-in functions - so your post is currently in wrong forum.

2. I'm not sure what you really did mean, but by current wording it looks like when p.e. total sale is 500$, and at 500$ percentage is 10%, then commission is 50$. And when Total Sales are 501$, and next percentage rate is 15%, then commission is 75$ and couple of cents too
I.e. at every new level, the commission makes a huge leap, and then continues at more moderate pace. Is this really how you want it? Or do you want the new rate applied only to part of sum which is over the new tier value?

Anyway, here is the solution which passes for both scenarios. Let us start with more reasonable one in my view.

Create a separate worksheet p.e. Rates. On this sheet, enter column headers like Tier, Base, Rate;
Format columns Tier and TierBase as money (or number), column Rate as percentage and set decimals as you like it;
Fill the table:
0, base0=0, rate0
100, base100=base0+(100-0)*rate0, rate100
150, base150=base100+(150-100)*rate100, rate150
250, base250=base150+(250-150)*rate150, rate150
e.t.c. NB! Value of Tier in table starts with new rate, but in reality the commission for Tier value equals to Base value.
When the table is filled, declare it as table and name it tRates.

Now, whereever you need the commission to be calculated (MyValue), enter the formula:

=INDEX(tRates[Base];MATCH(MyValue;tRates[Level];1)) + (MyValue - INDEX(tRates[Base];MATCH(MyValue;tRates[Level];1)))*INDEX(tRates[Rate];MATCH(MyValue;tRates[Level];1))

What about case you want to use same rate for whole sum? Simply set Base in Rates table 0 for all tiers.