I'm not sure where this cafe you can get waited on is selling a hamburger, fries, and a coke, with tax and tip, coming out less than $20, but if it's edible, definitely, I'd eat there if I had the choice. The difference between the cheapest sit down restaurant and fast food is almost down to nothing, but fast food is still the cheapest thing you're going to find, and the fast food companies know this, which is why they're comfortable raising the prices. They're attitude is if you don't want to pay our prices, eat at home, but we know you're not going to do that.
I regularly go to "La Mañica" in Albacete, where you can get the baturro with a drink and coffee for less than 10€. That bocadillo is bigger than my arm.