My one and only joke

I'm not good at remembering jokes, so this is the only one in my bag of tricks:

So there's this koala looking to get laid
The koala is attending the annual koala convention in Vegas and hangin' at the bar. While he's having a drink this really hot chick sits down next to him. They strike up some small talk and he buys her a drink. Things are looking good for the koala and she asks him if he'd like to go upstairs to her room. The koala says sure! Once they get in the room things get pretty heated. The koala goes down on the girl for some oral action. The girl really enjoys it and gets off. Once they're done the koala straightens himself up and heads for the door. The girl gets pretty pissed and says "you're leaving without paying me? If you look up prostitute in the dictionary it says: one who has sex for money." The koala replies "well if you look up koala in the dictionary it says: eats bush and leaves"