Charging for soda sends the bad message that we want you to pay for soda, rather than "we don't want to make your simplest hydration option very unhealthy".
That's why you don't charge much. A nickel or dime...hell a penny. Just enough to make someone think, hey maybe I should drink something healthier. The best system would be to make the first and maybe second soda free and then nudge people to water or some other drink.
Or just put some chilled water next to the Coke. There's just so much Coke you can drink, but if people are going to get all hairshirted about it, it rankles.
