For Snow Faerie Quests, you really shouldn't exceed 5k, because she usually gives 1k-3k in np + a random item + a random snowball.
For Kitchen Quests, don't spend more than 7k, cause sometimes he'll give 1k-2k in np or a random item or raise one of your pets stats.
For Edna's Quest you shouldn't spend more than 1k since she only gives out spooky food which is relatively cheap and the chances that she'll give you an expensive spooky food is really low.
For the Esophagor, if its worth more than 1k for a quest, it's not worth doing as he gives random items as well, mostly crappy items.
If your going to do a Brain Tree Quest, do an Esophagor Quest first and see if you can complete it without spending more than 1k. If you can, accept the Brain Tree Quest, then do it again, and atleast you minimized your costs. The Brain Tree gives out a spooky item + some np + a brain tree weapon.
Hope this helps