The west end is very expensive because the theatres tend to be old and small, the production costs are astronomical and running costs huge.
Even after the initial investmet a major West End musical can have fixed weekly running costs of £450,000.
Salaries for a large cast of actors, dancers, and singers, ( and probably a full orchestra/band), stage managers, technical crew, and front-of-house staff for (usually) eight performances a week.
Renting the theatre itself.
Ongoing upkeep of the elaborate sets, costumes (dry cleaning, repair, and replacement), and technical equipment (lighting, sound).
Royalties to the writers, composers, and rights holders of the show.
and then utilities, insurance, and other operational expenses.
This is after the £8m - 10m set up costs.