In the 60s they had pressurize tubes running to your table with the food. But the person putting it into the tube had to know what table your were sitting at. 70s came around and the process of just yelling out the person's name to come up and get it was found to be more efficient. Don't see how this could be more cost efficient than that - unless it is at a fancy restaurant. Then there are other jobs that the waiter does beside transporting food.
It will all boil down to what kind of maintenance is required. A robot for $50k would pay for itself in saved wages in under a year, even less if it collected tips. A lot of smaller diners (Waffle/Huddle/Waddle/etc) typically have super low staffing requirements (line cook + 1 or 2 servers per shift, occasionally more) and could totally use robots due to the simple layout and standardization of the restaurants.
The problem is that the wealth generated by the robots is going to the owners, not to the workers that lose their jobs, as always. Otherwise it'd be great.