I'm unsurprised people are unwilling to pay for this service: InboxCleaner provides very little added value on top of a service which is free.

Further: it's trivial for a competitor to undercut your app with a free alternative. I suggest you give this away for free to (a) build goodwill and (b) attract people to your other products.

Another thought: you mess with your pricing an awful lot. If I was your customer, and recommending your product to my friends, I'd be pretty pissed to find out the price has changed and made me a liar.