You need to look at using a field. When you do your attribute you need to call your Tag AREA then in Value insert field - Select Formula then rightclick in formula and insert another field. (confused yet?) now without looking i think you now have to select object (one of your distances) then * right click and insert another field then object (the other distance) Erm i think that should work i'll go check it!
I would then extract that data to an excel spreadsheet to work out cost.