This is the best way to take advantage of the writer's experience, and it may save duplicating work at a later stage in the project. The writer can research data, sites, locations - and people - to include useful information in proposals, scripts, drafts, and synopses. Research time is often costed by time, at a daily rate.
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.