Each app can be very different from the last, so there is no average app development cost.
We charge £300 per day, per developer. Small apps can take 4-5 days whilst medium scale multi-platform apps can take 15-30 days. At the top end of the scale, large enterprise, multi-platform apps that leverage online data can take 30-60 days to build.
If you have a specific budget in mind, let us know and we'll see what we can do. We can build larger apps in phases to ease your cashflow.
We are more than happy to provide a rough ballpark based on a vague description of your app, just drop us a line with your requirements.