Real exploits should be measured by the amount of gold they introduce in the system in an illegal way. The linen farm barely introduced any gold because vendor price of linen is a few coppers. Instead it had the opposite effect, it made gold get out of the system in the form of TP tax.
First, the vendor price of an item is not the "real" value of item. Rather, the vendor price it sets the "nominal" or base value of the item. The concept of "real" value in economics would factor in inflationary factors, but without knowing or having a rate of inflation to measure against that seems awkward for describing this. In addition, since the nominal value of items in GW2 is often absurdly low, a truer measurement of value for items in the game would be their market value. Even with TP fees factored in, an item maintains most of its market value when traded between players.
As for determining what is a real exploit or not, if we were to use this method of basing it purely on vendor price, then an obscure bug that allowed a player to easily and rapidly obtain several precursors would not be a "real exploit" by your definition despite clearly allowing that player to use an exploit method to obtain items of great value. To measure how much advantage one player would gain from such an exploit, the value of an item needs to be measured by the amount of gold that item can be converted into using any available method (vendor, TP, etc). As a result, it does not factor where that gold is obtained from. For example, if I were to find a bug that I could easily replicate which quickly gave me a stackable and valuable item I could sell on the TP, that would clearly be a real exploit, regardless of item's nominal merchant value. Each item I sold would be netting me a significant gain over other players that don't know about the exploit.