None of them have ever had a purchase price because, for a non-game (and even for a lot of games) that's the surest way to have no one download your app.
I guess since you're a developer, I have no choice but to take your word for it. But I find it very hard to swallow. I have spent a lot on apps for my iPad/iPhone. I wouldn't bat an eye if I had to pay $50 or even $100 for a good screenwriting app, or word processing app. Trouble is, that what's available is so poor at any price. Now, where I could see a problem is that the surest way for me not to download an app is if any form of subscription is involved. I'm not saying there's anything wrong with a developer going subscription, it's just not for me - I hate, hate, hate the feeling/knowledge that there's a "bill due" somewhere out there; in my life, in every way I can, I try to lower my overhead, cost of living etc. - which means never taking on payment obligations if I can possibly avoid it (some, like f.ex. insurance, you can't). It's not 100% rational, I admit, because I'm more than happy to pay what in effect might be the price of a subscription if it's a one-time thing - so I'll pay a one time $100 happily but not $5 a month. If a new version comes out, and I feel I need it, I'll buy again for $100 (or, say $80). I look at it like buying a hat - I buy it once and when it's worn out, I'll buy again, but I'll never rent a hat.
But hey, there are plenty of people who are happy to pay subscriptions, so I don't know if folks like me, enemies of subscription matter much.
I'm just surprised there to hear that the number of people who would buy an app is as small as you claim. Because I don't see it - the few friends with whom I discuss these things (we all use iPads), are all happy to pay, and I even know one guy who doesn't mind subscriptions. Maybe I live in a bubble, but I'm happy to pay - only unhappy that there's not much worth paying for. YMMV.