@MMW.... Does the URL contain a better explanation (captured in 2007; the webpage might be even older): http://www.mcgimpsey.com/excel/variablerate.html
(Caveat: That seems to be a slow link. Give it time to load.)
Thus, your table becomes:
Formulas:
C2: =B2-N(B1)
F2: =SUMPRODUCT((E2>=$A$2:$A$4)*(E2-$A$2:$A$4), $C$2:$C$4)
The addition of column C allows the SUMPRODUCT formula in column F to be normally-entered in all versions of Excel.
It is also more efficient because it removes a constant calculation. And it might be more self-explanatory.
How it works..... The first calculation (E2-A2)*C2 applies the first cost per unit to the entire amount (E2). In this case, that overstates the cost for weights above 201. So the second calculation (E2-A3)*C3 subtracts the overage for the __all__ of the remaining amount. But that still overstates the cost for weights above 301. So the third calculation (E2-A4)*C4 subtracts the additional overage for __final__ remaining amount.
PS.... You can also enter 0, 200, 300 into column A and change first term of the SUMPRODUCT to E2>$A$2:$A$4. I choose the form that fits the problem description.