I think he fought Briggs and Holyfield. Those fights each should have grossed him >$1m. Then the Wlad fight if I remember correctly would have been $4m gross. Moscow is one of the most expensive cities in the world, if he lives on the outskirts he can probably make a good living just on the interest alone, if he lives within his means and doesn't waste a lot of cash.
He can always go to the WBA and try and tap in to the neverending cycle of title fights with Valuev and Ruiz.
If he does come back I hope it's for the right reasons. Sultan was pretty good when he was champ but certainly not elite. Moy fondest memory was him brawling with Goofi at the press conference and then flying out of his corner in the first round unloading on that massive mid section.