$250 for each senior or $13 billion in total. It's bad precedent to go around a COLA calculation, even on a one-time basis, but you can construct a partial defense of the policy (here is Matt's semi-defense). Think of it as a helicopter drop of money, a'la Scott Sumner. If the helicopter drop substitutes for (part of) a second fiscal stimulus, that's a net gain. The drop of money stimulates aggregate demand, limits deflationary pressures, and, by the way, you're giving it to a lot of people who are not stuck in a liquidity trap. They'd love to buy more stuff in The Dollar Store.
How will the expenditure be financed? Obama was vague on that, but as usual the Fed moves both first and last in the monetary policy game. All Obama has to do is make the second stimulus $13 billion less than it otherwise would have been, wink and nod to Ben B., and it is all (or mostly) for the better.