I’m hosting a transmog competition on Halloween for my community, and I’m thinking that I’ll give a battle pet as a prize, since it’s something I can easily get to players that are on a different server. I’m not too in to battle pets, so I could use some recommendations on pets that might make a good prize.
Something that falls in the 50k-75k price range would be nice. A pet that seems somewhat Halloween-y would be extra nice. Any thoughts?
You can always do a search for Hallow’s End pets, which will fulfill your Halloween vibe 
Widget the Departed and Feline Familiar are both popular for various reasons. The Feline Familiar has the added bonus of being a pet you can use to work toward the Crazy for Cats achievement. Plus, it rides on a broomstick when you mount, which is why it’s one of my absolute favorites.
The Macabre Marionette is pretty useful, though I’m sad they still haven’t fixed its hat clipping issue.
Otherwise, you can just peruse the undead pets in the journal and see what strikes your fancy as long as they’re cageable. If you’re going for looks, then the utility won’t be much of a factor, so I’d say have fun with it.
If you have a good server for it, you might even be able to get a set of Hallow’s End pets in the price range you’re looking at. I think Widget the Departed will be the most expensive of all the cageable ones, though. On my server it sits at about 46k.
Please do Widget Kaanan lol.
In all seriousness I suggest going to Xu-Fu’s website and checking out their list of best pets for power leveling and top 5 most used battle pets.
I can do Widget. 
My suggestion is to make a toon on a high pop realm and search the auction house for all battle pets, sort by price and see which ones go for the most. They are a good indicator of which pets are currently “rarer” right now. You can also loo at the WarcraftPets website and check the stats on the lesser owned pets, which gives a a good indicator of which pets are more likely to be good rewards for all players. None of these is a perfect indicator in and of itself, but all good data points to consider.