I think you could simplify the concept in an elegant way by just counting the number of treasures (or cards) on your dragon mat. It's probably not very common that it would result in a different outcome anyway (people will be likely to have more Coppers than anything on their mat).
I thought about that but it would create an asymmetry between the Event and the Landmark.
The question you should ask yourself is whether you should kill a dragon to spend the money or keep the money to compete for the 4VPs. If the two values are different that becomes messy or trivial. If you had e.g. 3 Gold on your mat you certainly wouldn't keep them for Valyria if the Landmark only counted the number of Treasures.
I don't think that it is worthwhile to simplify this to "number of Treasure cards" just to deal with rare cases of Treasures that tell you to play or gain something. My rule, for the FAQs of the card, would be that as the game is over you cannot draw or gain anything.