Bitcoin production is supposed to be about 7200 coins per day, which at $13.75/BTC is currently $99,000. So, right now, it takes about $100,000 a day in new money to keep the price of Bitcoins stable.
We just saw this on the 4th of July. No one was putting much new money into the markets, since it's a US bank holiday, and the price dropped from $15 to $13. The "weekend slump" has been mentioned previously. But it's not because people aren't trading - that's symmetrical. It's because they're not depositing new cash.
How long will the supply of suckers able to collectively put in $100,000 a day hold out?
I'm not an economist but I think you got it wrong. Whats relevant is the supply and demand of the coins. If 10% of coins are in active trading and others are hoarded then you need money for only the 10% of the 'total value' (total created BTC * price in $) to have a stable price. That is if bitcoin trading is active with say 1,000,000 BTC (supply-side) and 15,000,000$ (demand-side) the price would be 15$/BTC. The rest of approximately 5M bitcoins of course have this same value but if you try to cash them the price plummets as supply increases. This is the logic of the overall price-production of bitcoin.
Mining changes the amount of total amount of bitcoins, part of which will become new supply to the trade and thus requiring influx of dollars to stabilize the price. If they all were directly cashed out, then your calculation probably would more accurate, although I don't think its even in this case that simple. Anyway, its not that bad, methinks.