Found it! And I got it to reproduce on my end. Here's the fix:
Your equation reset (in PHa) contains a subtraction of your summation (@I total cost). I simply set my variable to 0. What your's does is subtract the last calculation of "@I total cost" (from the previous page) thus putting your summation off by that much. Put this in to your equation for Balance_Reset:
WhilePrintingRecords;
Shared NumberVar BalanceDue_total;
BalanceDue_total := 0