Custom Members based on account type

I want to create a variance column, of actual vs. budget. However, it needs to be relative to the item. For example, for revenue, if the actual > budget, the variance would be a positive number, if actual < budget, the variance would be a negative number. For expenses, it would be the opposite ... if actual > budget, the variance would be a negative number, etc. Is there a way to write a custom member that does that?