There are a million ways to crunch the numbers. I think authors can break even when they sell 25-50 plans, depending on a million factors of course. The big upside is that, once they hit that figure, if the plans are good, their work is done after that point, and the 70% of revenue they receive for all future sales would be pure profit, with minimal (or even zero) work on their part.
It's also important to consider online shopping conversion rates - the percentage of people who visit a site vs. the percentage who make a purchase. Industry average is around 2-3% conversion rate for online shopping sites, so out of 100 visitors, 2 to 3 make a purchase.
If our conversion rate was just 1%, and all purchasers only purchased a single set of plans, we would sell 60 plans per day.