"If I just capture ONE PERCENT of this nine figure market, I'll be rich!" -standard entrepreneurial slide deck
The paid rate would be well under 1%. Potentially an order of magnitude lower than that.
Also, good luck getting r/Frugal to pay for that. This would be a horrible business model for Reddit.
1. Reddit already has competitors, and lots of them. By putting a 'pay to join' barrier up, they just send said competitors more users which gives sites like Voat a huge boost in popularity.
2. It's also competing with online forums and community sites, the vast majority of which are free. Will people really spend money to talk about say, video games or R/Gaming or R/Games when tens of thousands of forums for the same topic exist and are open to join for free? Perhaps not, which would give the likes of IGN, Gamespot and NeoGAF a huge boost among many others.
3. $12 is a lot of money for some Reddit users. For example, people in poorer countries/where the local wages are lower, people without a steady job or career and kids/teens in general.
4. Reddit's entire popularity is dependent on the work of its users. If they can't afford it or leave, then the site will lose out on future popular content, and likely enter a popularity death spiral.