0-500 $.55
501-600 $.65
601-700 $.70
701-800 $.80
801+ $.90

Each tier is exculsive of each other and must have a complete sum.

For example:

Total of 752 units would be
0-500 $.55 = $275
501-600 $.65 =$65.00
601-700 $.70 =$70
701-800 $.80 (752=52 units)= $41.60

A total of $451.60