> if you were simply to collect that data yourself, sell it on the open market, and spend a portion of the proceeds on the tax prep service, you would end up with profit
Hmm. Could you sell it on the open market? If so--if the margins for the ad-supported model like Google's are in fact as big as they appear--why isn't there a Google competitor who provides exactly the service you describe: some kind of opt-in system where they collect data (via, say, a browser extension), sell it to advertisers, and pay you a cut?
One generic answer to "why does the market not offer [some seemingly reasonable thing]" is inefficiency: maybe there's some cartel system at work where all major advertisers are hoarding the revenue for themselves. But I find that pretty unconvincing, since the whole market _seems_ to be otherwise quite competitive, and with low barriers to entry.
Perhaps a more likely theory is that if you were to offer a "we pay you for personal data" competitor, you'd face massive fraud--a la click fraud--in which attackers would pretend to be real users in order to get paid for searching (or whatever), and that the subsequent need for identity verification would become so burdensome as to eat away any profits.
Anyway, an interesting thought exercise, but I think one can broadly conclude that either:
1. There are real obstacles to paying people the "fair" price for their data, such that the current system is in fact fairer than it appears.
2. The entire market is unfair due to a cartel or similar (though like I said, I find this fairly unconvincing).
3. This is a great idea and you're the first to have it, so you should start a company that does exactly this. ;)
No?