An estimation of how big the market for habit apps is:
Habit Streaks downloads for habit and habits keywords: 3,000 per month
According to 2010 study on SEO results share amongst top 10 results:
These numbers surely are even more skewed towards the top ranks in app stores since each app presented takes up the whole real state of the screen. Data that backs this assumption would be the fact that 90%+ of traffic goes to the first page of Google results, the 2nd getting less than 5% etc.
A complete.com study reveals even more data to back that these numbers might be low. In their study they found the following results for CTR share:
So let’s say my ranking for my top ranking keywords was #5 (that is a high estimate). I am getting roughly 5% of the downloads for the relevant keywords.
3,000 downloads/mo = 5%
60,000 downloads/mo = 100%
60k/mo * 12mo = 720,000 downloads total
At a conversion rate of 4.5% (I was getting 5%+) we get 32,400 purchases annually. If we average out each transaction at a low $3.00, we $97,200 USD as the total market.
 My revenue per transaction is $3.99.