The least expensive option would probably be a cage that connects to an xpen you would get for a dog. You may want to look into getting the rabbits spayed or neutered, or else the rabbits will start to fight or breed. The cage size should be twice the size of the rabbit when they are laying down all the way, so probably about 2 feet by 2 feet for them now, but you will need to expand as they get bigger. Keeping them inside would be the best option for both the rabbit and you, because outdoor cages tend to be very small and also break easily. If you need anymore information on caring for those little guys, try visiting rabbit.org, or asking more questions on here. Good luck with your new companions!