Do you really expect somebody owning 6700$ in BTC would care for 1.7$ prize?!
I have a paper wallet with less than 1BTC on it, but it's not worth the hassle for the 10 cents you'd give.
and the 10 cents may cause you lose your 6700$. lolz... becareful folks...
Signing a message doesn't make people lose money, unless maybe it's done from an extremely buggy client
But the meaningful way to keep bigger amount of Bitcoin is in cold storage or paper wallets. And that means extra hassle to sign a message. And the reward is way to small for any kind of hassle.