A day late and a dollar short....

Didn't see the March 2007 challenge until it was too late, but here is my take...

In B10
=IF(SUM($A9:A9)>=$E$5,B9*$G$5,IF(SUM($A9:B9)<=$E$5,B9*$F$5,(SUM($A9:B9)-$E$5)*$G$5+($E$5-SUM($A9:A9))*$F$5))

Copy across to M10.

Lookup tables would be the way to go if there are more pricing tiers. I know Chip Pearson's website and a few others have progressive pricing formulas that will work here as well.

Yeah, I missed it too. I wasn't paying attention. As penance, I built one without using an if function.

In B9
=((--(SUM($A$8:A8)>$E$4))*B8*$G$4)+(--((SUM($A$8:A8)<$E$4))*(--((SUM($A$8:B8)>$E$4))*(((SUM($A$8:B8)-$E$4)*$G$4)+(($E$4-SUM($A$8:A8))*$F$4))))+((--(SUM($A$8:B8)<=$E$4))*(B8*$F$4))