Is the math that the dev did broadly accurate? If it is, it seems they could safely charge 10-19x less and get more revenue per-user than they do from ads alone... assuming that even a 10x lower cost would be affordable for third-party clients, which isn't a given.
>>solark+ea
I think it's more about the fact that the users are paying for the service. The % of reddit's userbase that has enough disposable income to subscribe is a very very very high value slice for advertisers.