the P.E.T. model does not imply anything about income level. It implies that withdrawal of reward (or of positive attention) can be a potent influence on child behavior.
Jeff does bring up an excellent point about age being a factor in being able to apply the method though. However I disagree that spanking is more suitable for younger children who don't quite grasp logical consequences. I think it is less suitable.
Well, I don't see how you can escape it, when 'rewards' imply something a child is getting/wants to get. Positive attention from the parent? Well, I guess that's OK if a child feels close to their parents and wants to hang out with them. Again, the P.E.T. model makes assumptions about a child's home life, both in income and relationship with the parents.
If a child is too young to grasp logical consequences, then talking to them, time outs/privileges lost isn't going to make much sense to them either.