Lots of reading can be found on this topic, no doubt. Angry Birds took like 8 months. You can find quite a bit on their own site, but here is a news article from last year that may provide some insight as to the process, expectations and unexpected/unforeseen issues.
Many apps have heavy backend, which creates huge costs. For example, the famous Viber (Telecom app), developed by our teams in Belarus, takes a solid group of engineers just to do regular support. There are also mobile apps for enterprises, which require linking with SAP or other corporate systems and may easily cost $1 mln.