I have multiple products that have multiple price tiers based on volume
ranges. For example, Product A may have the following:

1-100 units @ $10/unit
101-200 units @ $9.50/unit
201-400 units @ $9.00/unit

etc.

I need to be able to enter one number for the total volume of units I intend
to purchase per year and then calculate the average price per unit for the
year. Using the example above, if my annual requirement is 325 units I would
be charged $10/unit for the first 100 units, $9.50/unit for the next 100
units and $9.00/unit for the remaining 125 units.

What is the best way to calculate this?

Thanks in advance for any help!