The reason I willingly give my data to google by using gmail is that they give me an easy to use cloud mail with decent spamfilters. Or in other words, they are saving me time.
Maybe you could offer me something other than money? Like financial statistics on how I use my money? Sure, but those apps are already here, and people are willingly giving away their data without privacy in those.
So you see, it’s not that I don’t think you can’t convince people to give up their data, I absolutely do think that you can, I just don’t think petty cash and privacy are good motivators.
Also I think the reason you willingly give data to Google for Gmail is that you don't have a choice. It's take-it-all, the spam filters and the data mining, or leave it and use ProtonMail, no in-between.
Your personal data has a cash value only because it can be used to influence future purchasing decisions. It's a simple equation, $x worth of personal data has a y% chance of generating sales worth >$x. It therefore stands to reason that the market value of the personal data of a person who doesn't have as much purchasing power will be less, and may not even cover the cost of stealing and then processing it.
Would you also sell your data for $3 a year?
What kind really? Here's the thing many people touting blockchain forget things like cryptographically secured messengers are not the "must-have" of an average Joe. Is it easy to use and can I talk to my friends? These are the only two things matter. Blockchain, for better or worse, has made things even more cumbersome to use. You have people relying on good-old physically secured hard wallets or paper to secure their keys. This is not a sign of progress.
And frankly about selling your data for money. I think people will find it easier to have some utility like free email or apps in exchange than money through blockchain.