Run Business Rule on Result of Calculation

I have a Number whose value is set by a business rule.  I then need to evaluate if that Number is > another field already set and, if so, display an error. 

It appears that within a Business Rule one cannot evaluate a field whose value is derived from another Business Rule.  URGH!!  This seems so straight-forward of a need.  Am I interpreting this correctly, and how do I get around it?

Detailed scenario:

Fields:

• Balance Remaining = $200
• Total Cost = (Field A + Field B)

Business Rules:

1. Calculate Total Cost
2. If Total Cost > Balance Remaining, display error.