If I had to bet, I'd bet the animals would cost vouchers, not diamonds or coins. I have no evidence of that, just that the basic farm animals (without which the game cannot be played) cost coins, and the pets (which are very nice but not critical) cost vouchers.
If the test is: can the game be played without this animal, then an Animal Sanctuary presumably would not impact the fundamental operations of the game (using crop and animal products to make stuff to do Boat/truck/town orders). You can play hay day and ignore the Animal Sanctuary, in the same way as y can play hay day and ignore the town, the mine, the boat dock, and the truck board.
The animals in the sanctuary could be analogized to the pets we already have + the town. Therefore, I would expect the AS to operate the same way: animals consume farm products and give EMs, with different types of animals available at different levels. Visitors come to the AS from the town on a separate train, so I would also expect opportunities to upgrade the sanctuary train to bring in more visitors, and service buildings to accommodate their orders, which would give AS Points and EMs to upgrade the sanctuary.
All I can say is, I hope the AS will need MAPS (but I bet it won't). Again: I could ensure that the AS would utilize maps by selling off my map hoard, but I'm not going to do that, so this means we will all need a new kind of EM to expand the sanctuary. LOL and you're welcome
These are just my thoughts - I have no extra or inside information.