The problem is, there's no relation between value of the message to famous person, and price someone else is willing to pay to send it.
Just because someone's willing to pay $100 for someone important to read their message, doesn't mean their message is any more valuable to that person than someone not willing to pay money, or willing to pay only $5. Someone important isn't going to care about the money.
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If this is just for famous people...
The problem is, there's no relation between value of the message to famous person, and price someone else is willing to pay to send it.
Just because someone's willing to pay $100 for someone important to read their message, doesn't mean their message is any more valuable to that person than someone not willing to pay money, or willing to pay only $5. Someone important isn't going to care about the money.