Want to calculate breakeven for multiple products

It's not clear to me exactly what you mean by "break even" in this case, nor do I know how the "weighted average" method works for this sort of problem. Someone who is more familiar with these kind of financial calculations may understand what you mean without further instruction, but it would help the rest of us help you if you could explain the math behind the problem. Then we will be better able to help you find the programming solution to the problem.

I see two generic approaches to this sort of problem:

1) With the spreadsheet set up like you have it, identify the "break even target" (I would guess it is profit, but cannot be sure) and the "changing value" (number of units??), then use Solver or Goal Seek to set the target value to a value of 0 by changing the changing value.
Goal Seek help file: https://support.office.com/en-us/art...3-4f369d6e66c7
Solver help file: https://support.office.com/en-us/art...d-078f96d1652c

2) Algebra (which is mostly done outside of Excel). Write out the formula for you target value, then set it equal to 0 and solve for the changing value.