There are 6 remaining millennium problems worth 1 million dollars each.
Between 2000 and 2002 Faber & Faber offered the same amount of money for proof or disproof of Goldbach conjecture (by the way: why did they cancel their offer in 2002 - were they afraid someone might solve problem and they'd actually have to pay the money if they waited longer?)
There were Eternity and Eternity II puzzles, solutions to which were worth 1 million pounds and 2 million dollars respectively.
What about the rest?
There must be many more opportunities to make serious money by solving mathematical problems. Could you list some of them (with the amount of money offered for solution)? I'm interested in both "scientific" (open problems) and recreational (puzzles, riddles, online games maybe) mathematics, I'm not interested in gambling.