a different approach:
total sales in cell b11
cells a1:a4 are your commission break points - 0, 30000, 60000, 90000
cell a5 is =IF(B11>A4,B11,0)
cell b1 is 0
cell 2 is =IF($B$11>A2,A2,$B$11) - copy this down to cell b5
cells c1:c5 are the commission %'s 0, 50, 65, 80, 90
cell d1 is 0
cell d2 is =IF(B2=0,0,IF($B$11>B2,B2-B1,$B$11-B1)) - copy down to cel
d5
cell e1 is 0
cell e2 is =d2*c2 - copy down to cell e5
avg % is sum(e1:e5)/b11
this allows you to easily change the break point