FRA pricing — hourly, fixed, per premises type?

Pricing FRAs is one of the trickiest conversations with new clients. Too cheap and the perception is you are cutting corners; too expensive and you lose work. How do you structure your pricing? - Hourly with an estimate? - Fixed fee per premises (tiered by size)? - Bundled with remedial advice? - Retainer arrangements for multi-site clients? I would also be interested in how you handle scope creep — premises that turn out to be far bigger or more complex than the initial brief.