A billionaire can back the R3VOLution and other pro-Paul organizations. They can donate $2,300 to his campaign. They can form other legal mechanisms to get money to Paul, but it still amounts to frustratingly small change for a billionaire. The only way the Paul campaign could legally get enormous dollars from this individual would be to have them as his Vice Presidential candidate, and that would only occur (I presume) after Paul wins the nomination or withdraws to make an independent run.