The way I see it: unless you’re putting in a bunch of monthly resources for me (in terms of content creation or server resources), you have no business charging me a recurring fee. If it’s essentially an offline app (or easily could be) then I’m either paying up front or not at all. I’ve passed up on many apps that I would have happily paid for (in some cases up to about $100), but was unwilling to pay a subscription of any cost.