for a suggestion which makes sense it would be helpful if you could share screenshots of both apps to see their structure (the screenshots should show the fields needed for the calculation).
1. I assume the month budget fields are money or number fields - correct?
2. Are the apps where the monthly budgets are in and the Expense Reports app connected via a relationship field?
3. Do you use Globiflow (it's included in the Podio Premium Licence)?