You should not go into a PhD program in any field if you're in it for the money. This is because your department will have at least a few grad students who are in it for the love of the field, and they will work insane hours. They will have no life, and will be fine with that. They will be nearly broke and will be fine with that. You will be in competition with those people (for thesis advisors, access to equipment, grants, etc) and you will have a very miserable experience.