edit: typo
I'm much more willing to buy a $15 app if it feels like it has legs even if I may give up with it after a month or two than to possibly lock myself into a much smaller amount forever.
The amount I'm willing to pay on the app store only goes up if the app is _also_ open source.
While a one-time fee seems preferable from a customer-standpoint, I personally prefer annual subscription fees. It allows for a potentially lower initial cost, making it more accessible to potential customers, but--more importantly--provides an obvious recurring revenue model for the author, hopefully implying on-going development. Ideally you would retain access to the app if your annual subscription lapses, but could receive upgrades after reviving your subscription. Whenever there is a one-time fee for an "average" app (no offense intended), I always wonder how the author can afford to keep updating it in the future.