Feudalism, such as it is, in "A Song of Ice and Fire", is an interesting concept when peasants are not bound to the land. There was no Westerosi emperor Diocletian. The great houses operate very much like the old kings of Ireland, and India during the Raj also. In both cases the English monarch became the overlord of these peoples in the same way Aegon the Conqueror conquered six of the seven kingdoms. The Lords are petty kings with the iron throne belonging to an overlord (or High King as was the equivalent case in Ireland). I would assume that every local lord is able to demand some from of tribute from nearby peasants but even this may not be certain. What the main lords subsist on, I would imagine, is their ability to enforce the king's laws and their rights over the lands they rule. This specifically means being able to fine poachers (and criminals in general). One could imply from the television adaptation that each of the main lords depends on their control...