This formula assumes all commissions are 3%, as set in cell H4, the named range comm_rate.
The commission amount is split between the agent and broker, starting with a 50/50 split, and changing at fixed intervals as shown in the named range split_table (G7:I11).
The commission amount is calculated in column C with this formula:
=B5*comm_rate // total commission
The Agent and Broker amounts in columns D and E are calculated like this:
=C5*VLOOKUP(B5,split_table,2,TRUE) // agent =C5*VLOOKUP(B5,split_table,3,TRUE) // broker
Notice these formulas are identical except for the column index. The formula for agent returns a value from column 2 in the table, and the formula for broker returns a value from column 3 in the table. In both formulas, VLOOKUP is set to approximate match explicitly, by providing the optional fourth argument as TRUE.
When doing an approximate match, VLOOKUP assumes the table is sorted in ascending order. If VLOOKUP finds an exact match, it returns a value from matching row. If VLOOKUP encounters a value greater than the lookup value, it will return a value from the previous row.