The economics in this regard are the same as for gold. With gold only the most efficient gold mines can make a profit, so only those profitable operate -- nobody tries to mine all the gold in the world. With bit gold, only those with the most efficient processors and most spare time will be able to make a profit, so again there will just be a handful of a few powerful computers, or many small computers but just using the spare CPU cycles (like SETI@home), solving the mathematical puzzles to generate the bit gold.