The most expensive part of keeping any animals will be their "home" and it is worth it to not scrimp. I understand having a limited budget, so most of my lumber was from building site dumpsters (with permission to pick) or my local lumber yard with their pile of "not so great quality" lumber. (I work at the lumber yard now.)
Many years ago, I drove to sites where they were building homes, in my jalopy car, and asked for any left over plywood, and they gave me more than I needed to build rabbit hutches and a whelping box for puppies. Just look around and ask, you'd be surprised at how many sources there are.
