I don’t have concrete numbers either, so take this with a grain of salt (heh), but my experiences working with chefs is that it generally does make economic sense - and when it’s on the margin, most chefs also have a bit of professional pride that will tip the scales. Labour in the restaurant kitchen has an element of the “on-call” dynamic: you have to have enough hands in the kitchen to handle the peak (which generally lasts around 2 hours) but there’s downtime before and after the peak where they do prep work. Stock fits pretty neatly into ”start it before lunch service and finish it after dinner service”, for example.
One place I worked had buffalo wings on their snack menu, a fried chicken option on their burger menu, and chicken breast schnitzel as a main - the head chef’s costing showed the cheapest option was “process whole chickens: wings for the wings, deboned thighs for the burgers, deboned breasts for the schnitzel”. Put a 20 gallon pot instead of a garbage bin next to whoever’s processing the chickens and you get the meat for chicken stock for free. The wings, burgers, and schnitzels didn’t always sell at the same ratios so you’d sometimes have an excess of one of them from yesterday; that goes in the pot as well. The veggies don’t take long and the pot goes on a burner all day. Any chef walking by during lunch or dinner service skims the froth. When dinner service is winding up, pour it through a strainer to store. (Make sure you do this before the dishie is done for the evening so you can make them clean the pot.)
Because it was so integrated into other necessary processes, it was almost free, and they found a lot of uses for it.