Anyway to promote our app in the "pool of apps"?

I'm new to marketing.... really out of the idea how to improve the sales.

We have released our Expenses Tracker app which can be found at the following URL. But, the download dropped to zero after switching from free to paid. This sounds like the app got a better ranking when it is free and after I have changed it to paid-app which resets the ranking.

[ Link Removed ]

Is there anyway I can promote this app in the "pool" of expense tracking app? Using promo code, pay for app reviews,..?

Appreciate any comments.