I'm assuming one row is one sale in your sales data. If that's true, I think it would be easier to use nested IF formulas for the commission calculation. The formula would be like the below, where cell A2 represents the price of the item sold in that row.

=IF(A2<=600,1%,IF(AND(A2>600,A2<=900),2%,3%))

The $600 and $900 cut-off points are entered directly into the formula , but best practice would be to reference cells containing those values so they can be easily changed by anyone.