I actually have a dog crate that I am using for my bunny! He is a dwarf and the cage was used for a medium sized dog. It has the plastic bottom tray that can be removed, a top door and it works just fine.
He has all his things inside and comes out only when I am home. No need to be one of those crazy pet people who spend several hundreds on the rabbit cage just make sure it is sturdy, clean and no place for your bunny to get hurt.