Compare sales order final value to initial value

Hi,

We need a search to identify all orders where the final value of the sales order is higher than when the customer initially placed it.  This would happen because they contact us to add items to the order after they initially placed it, but before we sent it.

We currently have a search that identifies orders with a change to the initial value, but this also picks up orders where the final value is lower than the initial value, i.e. they have removed items.

We've tried using numeric formulas to achieve our aim, but haven't managed to get anything other than error messages.