At 2 lb/day gain, the 140 lb difference between 360 and 500 lb calves is 70 days more grazing of a calf on summer grass (in addition to still nursing the cow). What is the incremental cost of 70 additional days of that?

Feeders may not care what it costs to raise a calf, some cow/calf producers may continue producing calves despite losing money on each calf (the definition of a hobby) but an industry that depends on its essential raw material suppliers operate at a loss is not very stable long term.