The medieval European dragon has roots in popular culture where the creatures manifested in oral narrative and belief. In legends (stories told generally to be believed) dragons were generally understood to be creatures that lived in a remote past or in remote geographic regions. They were believed to be real, so the dragons of legends were not a cautionary device. They were just dangerous creatures.

Dragons also appear as monstrous antagonists in folktales, stories told as fiction, the novels of the folk. I don't believe I have ever encountered evidence of a European folktale being told where the audience or the storyteller understood its components in allegorical terms. The European folk mind did not think in these terms, so one would not look to form of oral narrative - legend, folktale, or others - to find evidence of this interpretation of the medieval dragon.

The dragon also featured in medieval and later literature. This is out of my field, but I do understand that the dragon often was seen as a manifestation of the devil (or a devil), to be killed by a saint (preferable Saint George). In the case of St. George killing the dragon, we see literary and often artistic adaptations of a famous folktale (ATU 300), taken into hagiography here the fight between hero and monster were seen in strict, Christian (and allegorical) terms. In this case, however, we are still not seeing the dragon as an allegory for the ultra rich.

Perhaps a medievalist can address this question of medieval literature and come up with an example along the lines requested by OP. I doubt, however, that such an example would represent the mainstream.