The BBC News vanity project, EU Referendum: The Great Debate was watched by a very healthy average audience of 3.86m (19.3% share) according to the overnight ratings.
The costs will be an FOI secret, held for the purposes of journalism. The hire of the SSE Arena, the audience vetting and security, giving the main panellists three mics (Madonna-style, lapel and desk) each (while Mishal Husain wobbled a single mike at her group of mini-mes), the spin-room refreshments, etc would probably add up to the salaries of three or four solid journalists for a year. Would it have been different, editorially, if the audience had been 600 rather than 6,000 ?
Still, the main panel had an admirable gender balance, if, from the observation of a few sweeping arial shots, the audience was BAME-lite. Big cheese James Harding took the pleasure of doing the audience warm-up; his Newsgathering boss, Jonathan Munro bagged a credit as "Series Editor", whatever that means.