It is absolutely acceptable! Actually I think there was a thread about this recently. One moment!
Edit: http://ball-pythons.net/forums/showt...ghlight=haggle
Found it
Most breeders have a minimum price they are looking to get for every animal. As long as your offer isn't like, way under that, they usually accept offers. I know I haggle most of my animals. I haven't paid full price in quite a while.