How do I get an app developed?

Hi,
I'm working for a company in the UK that wants to publish an app based on a book (so pretty basic, but it has to look sleek). We're a small company and we haven't worked in this field before. We want to have a good idea of what to expect before we start spending money on the process! Could anyone give me some (if any) replies to these questions?:

Getting a third party to publish:
- How much does it tend to cost for single/cross-platform? (UK)
- What's the time frame?
- Who's the best to use?

Hiring a single developer:
- How much does it tend to cost for single/cross-platform? (UK)
- What's the time frame?
- Who's the best to use?
- What skills and experience do they need?
- What resources will they require?


Any other tips/pitfalls?

We're not looking for developers to let us know they're interested, since we don't have definite plans for publishing the app yet.

Thanks guys!