With
H1: YTD_Pages
H2: a YTD count....eg 751

I1: CurrMthPages
I2: a Curr Mth count...eg 700

J1: Price

and...these values in M1:P4
Over       Thru    Rate  Incremental
0          750    $0.00        $0.00 
750       1500    $0.03        $0.03 
1500  Infinity    $0.05        $0.02
This formula returns the Curr Mth Price:
J2: =SUMPRODUCT((H2>$M$2:$M$4)*(H2-$M$2:$M$4)*$P$2:$P$4)
-SUMPRODUCT(((H2-I2)>$M$2:$M$4)*((H2-I2)-$M$2:$M$4)*$P$2:$P$4)
In the above example, the formula returns: $0.03
because during the current month, the user crossed the $0.03 threshhold by 1 page.

Is that something you can work with?