Because your knees are clearly not worth investing in.....
I don't understand how people can justify buying an expensive ski and then putting a shitty binder on it. Your connection with your skis is your chance at keeping your knees intact, and new binders will do that more effectively than will old or used ones. Your decision to use a look/rossi is a good idea, but most binders in the price range you suggested are best used for urban.
Look at it this way: Either invest in bindings now, or invest in knee surgery and a new couch to sit on later.
BTW, this may be an effective argument with your parents.