Custom column field to show item base price

Hi,

I have a custom field on my items which contains an items original full price (we often mark an item down permanently).

I can easily show this as a column value on my transactions, but find it impossible to show the 'saving' when comparing this to the price paid on the transaction.

I have tried a custom column field 'saving' formula where = custom column 'full price' - item price.

This display nothing; it seems impossible to get a formula to calculate on the items price.

Any ideas on how to calculate and show this column on a transaction?

Thanks