This is the usual way of working. The writer is briefed on what is needed, and is given the data he needs. He may produce one or more drafts for discussion before final approval. A fixed fee is often negotiated for the whole job (plus expenses!).
The complexity and scale of projects varies so much it is difficult to predict costs before a briefing. Daily rates may fall between £300-£500 or more: a minimum charge for small editing jobs would usually be £100.