I believe just about everything in life is negotiable.
If a coach offers you 25% and you see yourself as being worth more than that, you could say something like "Coach, I am thrilled that you even made an offer. I see myself as at least a 50% guy (or other number) and thus would like to wait for a better offer." He can say good luck or he can make another offer. If you tell him you are insulted by his offer (or some other non-gracious remark), it is more than likely he'll be upset.
The key to negotiation is that you have to be willing to walk away from the offer if it indeed does not meet your expectations. Are you willing to walk away from it and continue "playing the field?" Remember, the offer he makes is relevant to his program. He may only have 25% left to offer and thus it is the best he can do at this time. He might also come back with, "If you are the performer we think you'll be, we will increase the scholarship over time." If tactfully done, negotiating should be fine. That is how life works imho.