It also depends on what you're defending from. If the attacker is, for example, just after your wallet, all you have to do is raise the cost for him to get your wallet. That will probably be enough to repel the attacker. Antis, however, do not want your wallet. They hate you because of who you are. If you repel them once, they will not just drop it, they will come back again and again and again. Therefore, a whole lot more of force is needed.