While it’s a noble thought, I think your calculations are off by at least an order of magnitude.
Your surveying method was a Twitter poll of your own followers… this means that the only people who saw it were actively reading Twitter at the time and engaging with it. This alone represents a small and shrinking population of Twitter. Plus, I imagine that your followers are more tech and media saavy than others, and probably have more disposable income.
I don’t know much about Twitter’s actual demographic breakdown, but I’m pretty sure that if you asked that same question of a truly random selection of Twitter users, you’d find that the vast majority wouldn’t say that they were willing to pay. Plus, from experience, everyone knows that it’s easier to get someone to say they’d pay for something compared to actually getting them to pull out their credit card.
That said, I think there’s definitely revenue Twitter can be squeezing out of it’s most active, engaged users. I personally would pay more than the proposed $12 a year for a Tweetdeck client alone! But at the end of the day, we’re not Twitter’s market, and they can’t just bankroll the company off the back of tech/media folks like us.