Oh nooooo - this is awful!!! He completely took advantage of you. I know you - and you're probably avoiding talking to him like the plague!!! LOL but...if you are going to pay it - maybe you could say to him. "Oh! We never talked about a price so I was shocked at how expensive it was. Had I known you were going to chop down the tree, I would have stopped you since "neighbor" offered to do it for free! Is there some way that we can meet in the middle on this bill? I wasn't expecting this at all"

At least you probably won't pay it all? I don't think you should pay ANY of it.... but.... i know you ;)