How about just say that each pet should have at least $5-$10 for each? Like if they have two cats and a dog or two dogs and five cats, you spend maybe $7 or so for a toy or two the dogs can share and then another $7-10 for the kitties?
Like as a more general ratio rather than "Oh, twenty dollars min. If they have one cat, that cat gets a CRAP LOAD OF STUFF to get that tweny dollars"

And if someone feels like spending more, good for them, if others cannot or will not, it's not a crime, it's just spending less money. There are GREAT things that cost less than other items.

BTW: I'm sending an email now with my info on it. This is my first time doing this, so if I make a mistake please tell me ^^'