It's an interesting concept, and I'm curious what results you find.
Here's my issue:
You say you value a $0.99 app as throw-away, and that the more you spend, the less throw-away it feels. Sure, if you priced your app at $20.00, I agree that it would feel less throw-away (if a customer decided to buy it, they value it at >= $20).
But you're not pricing it at $20.00. You're pricing it at $2.50-$20.00. Really, anything I pay above $2.50 is a donation to you, for your work. But in my head, it's still a $2.50 app, isn't it? Because other people are paying that amount even if I'm not.
I think what you're talking about is setting my expectation of the value it will add for me. For a $0.99 app, the creator thought it would typically only add $0.99 of value. For a $5.00 app, they thought it would add >= $5.00 of value.
It just sounds like you're not sure how much it's worth (or you have customers who it's worth $2.50 to and customers who it's worth $20.00 to, and you can't choose who to target).
One more point/question... wouldn't it be better for them to "pay-what-they-want" after they try it? Before they use it, they can only guess how much value it adds for them. Are you expecting that they've already downloaded and used your trial, and then return to the website to buy?