It’s expensive, but it’s not overpriced because putting on a show in the west end is a hugely expensive business.
Once the initial costs of putting on a show are over there’s weekly running costs. For Matilda it’ll be something like-
Hire of the theatre (the producers of the show and the bricks and mortar owners are usually different entities)
Running maintenance on set and costumes
Hire of lighting equipment
Hire of sound equipment
Hire of automated staging
Staff wages, this will be cleaners, bar staff, ushers, box office staff, theatre management, stage door keepers (these may be included in the theatre hire fee) then show staff which will be Stage electricians, follow spotters, stage crew, flymen, automation operators, stage management, wigs and wardrobe, sound operators, understudies, main cast, band and associate directors who run rehearsals, of which there will be many because of the young cast.
The producers at the bare minimum to keep the show running have to break even. Most west end theatres seat between 500 to 800, and won’t be full Monday (if they open Monday) through Thursday outside of holiday weeks.