Use up the store credit
Since the Apple App Store has standardized price points ($0.99, $1.99, etc), you won't find an app that is $0.80 unfortunately. 

There isn't any way to search by price in the app store, but services such as AppAdvice.com and others can help a bit.

My recommendation is that you choose an app that you want, figure out the price, including tax. Ask a friend in the US to purchase an App Store gift card via the App Store app, in the amount that is the difference between your app of choice, inlcuding tax, less your $0.80. This friend will email you the gift code, which will load into your gift card balance. Purchase your app, and you have zero balance.

Perhaps even easier: purchase an $0.80 gift certificate for a friend in the US, and email it to them.