Microsoft Office Store is a great resource. Many interesting apps are showcased there. Some of them are free, some require payment and can be purchased once or on a subscription basis. First of all, Office Store is targeted to be a place for business apps. If an app helps you to run your business it is unlikely that it's price will be $1 or $10 as we can see it in Apple App Store or similar markets. The max price of a SharePoint app in Office Store is restricted to $999.99. So what price is fair for a SharePoint app?