The main site was always free and that became kind of a lead generation / acquisition funnel to have people sign up and become paid members.
What they pay for is a chat group and forum (and some extra apps I made for digital nomads, like a trip planner on http://NomadTrips.co). The price now is $75/year.
I simply started by adding little features (like a chat), I charged $5 one-time first, because I was getting a lot of spammers. Even with $5, people kept joining. So I kept raising it, to $25, then $50, then $65, and then in April this year I made it annual and recurring. Sign ups have remained the same and even grow.
Meanwhile the main site that's free (https://nomadlist.com) keeps being covered in mainstream press like Time, Times and HuffPo. I don't do any PR or marketing. So my CAC=$0. It also ranks very well in search as it's linked to by so many other sites. Also not actively worked on that, it just happened because people liked my site.
I've tried FB ads last month but only saw 4 conversions.
So I think organic acquisition works best for me. Which means: Make a cool site, add paid features, make it recurring. Win.
This is B2C though, so probably quite different!