Well, you've already got mercenaries in the game... why not give those that donate $2-3 one of them as 'bonus' to the dp. It's not as good the Kyuubi, but it keeps them interested. And, every bit helps those who need it and don't know the game as well as others.
It can even be as simple as $2 get you 1 nin and $3 gets you 1 nin and a medical nin to assist you. Essentially, you get 'stackable' mount/help. Now, how long they stick around can be determined like the Kyuubi (same amount of days or less).
That's simple enough without changing anything to drastically.