Typically cooks make the most money per hour. Prep cooks the next highest, followed by expoditers, cashiers, and hosts. Bartenders typically make above minimum wage plus tips.
Servers are the only ones not making minimum wage or above(except in certain cities/towns, Las Vegas is an exception).
I've never heard of an employer having to make up the difference to get them caught up to minimum wage if they don't bring in high enough tips. I can easily check on this though and update next week after the holiday.
A server is being paid AT LEAST 4 dollars an hour less than pretty much any other person working in that restaurant. Because of the structure of the system, and the implied understanding that tips make up for lake of wages, a restaurant does not have to factor the additional wages into the overall costs of the menu.
When you have a wait staff of 15+ servers per hour, that's at least $60 per hour that a restaurat doesn't have to cover. Assume you average 10 servers on duty over the course of a 13 hour business day (11AM to 12AM) and that's just shy of $800 a day that the restaurant doesn't have to make up. $780 a day, $23,400 a month, over $280,000 a year...and that's just assuming they are only making $4 less an hour than anyone else.
You do the math. That's a good chunk of change that the restaurant doesn't have to turn around and bill you directly for through raised food & drink prices. If you don't like how the system works, DON'T GO OUT. Plus, if you truely feel that you are getting bad service you can talk to a manager and any one of them worth a damn will go out of their way to make sure you leave happy. Comp'ed meals, free dessert, coupons for future use, ect.
Everyone has run into a thankless server or one that's a grouch. Assuming it's a well run restaurant, that isn't the norm though. If you are going to be cheap, eat at home or go to Subway.