I would say 9 figures for a single event is a ton of money. Especially when it's taxpayer money spent on a hereditary monarch.
The only way this is justified is if the event somehow raised more money than it cost, which I find hard to believe. At least you could point to how lucrative pro sports are to explain athletes' salries.
It's a huge event though. Hiring security, purchasing materials, equipment and supplies, logistics, rentals, and while people think of it as a 1 day event, it requires months and months of planning. I mean, the last coronation happened how long ago?
I mean, comparing it to a US Presidential inauguration, which takes place every 4-8 years is $100 million+, you'd think a coronation of a King or Queen in England which very rarely happens would be a substantially more expensive event.
I'm no fan of the monarchy, but the cost really isn't that shocking when considering the big picture.