Things may have changed since I left the travel business a couple of years ago but
-if you are talking about making sure valuabel luggage on a flight is covered then most airlines ,save the crappy ones, had an "excess value " cover for which one paid extra. sadly, not every employee of the airline knows about ti ,but it was a regular but little known eextra that most ailrines gave and probably still give. Ask about it.
-if it's luggage generally on a holiday whether in a hotel, train ,car then i never heard about it. Some of my richer customers got on to their household contents insurance people and had varying degrees of luck ensuring that their luggage was insured up to about twice the usual amount.
- check your credit card insurance.