I think what he means is -
He has orange robe X with no mods in it, so can see the very basic armour value for this piece (say 25)
Armouring mod Y is for sale, detailing an armour rank (e.g. rank 30)
Non-modifiable robe Z is for sale, displaying its proper armour value (say 210)
How can he calculate the value of X + Y in order to compare it to Z?
E.g. Say the rank 30 piece above provides 300 armour value
25 + 300 = 325, so he would be better buying the mod than the new robe.
(Disclaimer - These numbers are completely made up just for example purposes)