You know, in a way, smiling can be worth cold, hard cash. The attitude you bring to your job can absolutely affect how your coworkers and your superiors perceive you. And that might just translate into better pay one day.
Okay, I don't have the science on this, but positivity really ought to pay off, don't you think? It often does, at least in other ways, like making day-to-day living more pleasant for everyone. That has to be worth something!