Depending on your potential consumers and the reason for developing your app you can expect a variety of outcomes and sales. For example if you have a Business to Consumer app, you would expect margins to be smaller but potentially higher volume of downloads. If you are looking at building a Business to Business app, you would potentially expect higher sales margins but possibly fewer sales.

A lot of companies are realizing the inevitable migration to mobile content ingestion and in some cases would just like an app simply to promote their services in which case you would not expect a financial return but rather strengthening brand loyalty, for example.

In general, it seems that it's getting harder and harder to turn a profit on apps with a staggering stat that suggests that roughly 60% of mobile apps do not generate enough money to break even (source article). Don't let this put you off as you move forward but ensure your financial expectations are realistic.

Be clear about what your expectations are in relation to your audience and the general purpose of your app.