i imagine the cost will also include the two receptions held by Charles for Commonwealth Leaders and Foreign Heads of State at Buckingham Palace - two major diplomatic events that the Foreign Office will be delighted to have to opportunity to organise as a promotion of UK PLC.
Plus images of London and 100 years of heritage was front page news and top of the news feeds worldwide. Countries spend far more than the cost of the coronation to try and achieve that kind of marketing and international awareness. Samoa recently spent $40m to host the Commonwealth Heads of Government Meeting, for example.
And, as others have said, the money didn’t just disappear. it was spent on salaries, commissions, purchases - directly into the economy, in other words. Documentaries made at the time about, for example, the uniforms that were commisisoned made this very clear.