Offering a bounty means you are willing to give up some reputation for the good of the community, as all reasons eventually lead to better content or rewarding existing good content.
The act of offering is what matters most in my opinion, not manually awarding and we already got [investor][1] badge which is a good start - but I would like to see silver and gold badges as well.
Philanthropist - Silver badge. Offered at least 5 bounties worth total of at least 1000 reputation.
Good Samaritan - Gold badge. Offered at least 50 bounties worth total of at least 5000 reputation.
The limitations will protect the badges from gaming by offering minimum amount of reputation.
Edit: just noticed there is already pending feature request:
Can we have a badge called "big spender" or "seeder"?
[1]: https://stackoverflow.com/badges/219/investor
