Rational actors are more of an assumption then a requirement. Used in conjunction with carefully defining terms, this a powerful way that economists hermetically seal their theories.
rational = utility maximising
utility defined by consumer choice
Basically, they pay anything because it's worth anything.
All this really means is that preferences are transitive. I.e., if I prefer a puppy to a wii, and prefer a wii to cabbage, then I prefer a puppy to cabbage. That's not an unreasonable assumption.
It's an easy topology exercise to prove that if preferences are transitive, then a utility function exists (I think you may also need to assume a countable number of goods/services).
rational = utility maximising utility defined by consumer choice
Basically, they pay anything because it's worth anything.