Formula for budgeting from lowest cost unit

Hi all. A workbook is attached. What I am trying to achieve is to use the budget given each year to remove the pollution. Each year the budget should be spent to remove the pollution from the location where the unit cost is the lowest. A screenshot is attached for the result (partially) I intend to get. What is the most efficient way to use formulas to solve a problem like this? Thank you very much for helping.

one option:

Formula:

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refer attached for working example which should translate per your locale settings