The volume pricing model allows you to define multiple tiers, similar to the graduated pricing.
The unit price then depends on the total volume of units.
For instance:
Tier 1: 0 to 100 units - $4 per unit
Tier 2: 101 to 500 units - $3 per unit
Tier 3: 501+ units - $2 per unit
If at the end of the billing period the total volume is 400 units, then the second tier will apply and the charge will be 400 x $3 = $1,200.