So you have 521 pets in your stables at 60% so maybe the discount increases 10% after each 100 pet you have. 0-99 pets 10% discount, 100-199 20% discount, 200-299 30% discount, 300-399=40% discount, 400-499=50% discount, 500-599=60%, 600-699=70% and then 700 or more is 80% discount. That may not be right though.

I have 551 pets in my stables (not counting the teens in my hatchery) so I know I am at least elligible for the 60% discount. So the total cost for me for 84 days would be 471,240 gold plus 100 diamonds, rather than 1,157,100 gold for 84 days to feed them manually.