What would make you a bitch would be appearing to go along with this plan of dividing the prize money then backing out if you win. However, telling them straight up that you don't want to be part of such a scheme is not being a bitch- it's looking out for your own legitimate interests. If you did win, though, it would probably be the nice and polite thing to do to treat your friends to dinner or drinks.

I'll agree with others that refusing to split the money, even if you gave advance notice, might piss your friends off, though I still wouldn't call it being a bitch as such.