[thiagodotfm]

Charge yearly a X%(go figure it out what works best) of what you would charge one-time.

Charging a one-time fee is evil for the customer with common sense. If your service stops growing, what about the customers that already paid for it?

You don't pay for updates in a SaaS app as it's in the cloud, so... I can only truly see a recurring model working. UNLESS you know very well your userbase and you know very deeply it's the only model that would work.