I would highly suggest checking out Craigslist for used options. Maybe also Ebay and even garage sales. Depending on how much the pen is out of your price range, you can also ask your local pet stores if they have any upcoming sales going on.
When I first got my bunny I had him in a similar cage and let him run around my studio when I was at home. He is litter trained and neutered, and doesn't make messes. I wire-proofed and bunny-proofed my whole apartment, and now he runs around freely 24/7. No accidents yet...knock on wood! I was able to return the cage I was keeping him in (actually it was a dog crate) and a lot of the things I had gotten for him, so I saved money also.
Good luck with your bunny's home - he is adorable!