Calculating a blended rate

I'm looking to do something similar to sliding scale commission but with fewer variables. Attached example worksheet, which goes like this:

Cells D2:H2 contain threshold numbers ("buckets") (2,000, 4,000, 6,000, 8,000)
Cells D3:H3 contain dollar amount per unit in each bucket ($200, $180, $140, $120)

I want to calculate the total cost for a number that falls in between two of the buckets, for example 2,500

The first 2,000 are multiplied by $200 and the next 500 are multiplied by $180.

How can I write the function most simply?