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.