I remember reading something saying that average Broadway musicals cost $7-9m to opening night, and that West End shows cost slightly less, but that was about 7 years old, so you'd need to adjust that significantly. I still think Broadway ones cost more.
Costs to Opening: (Copy the list and add expenses you can think of!
Rehearsal Space
Set Construction
Design Set/Costumes/Sound/Lights
Theatre Fit-Out
Transport
Rehearsal Wages for Cast, Crew
Production Team Fees
Advertising
Orchestrations/Music Preparation
Props Bought
There's a breakdown in Richard Andrews' Guide to London Theatre handbook. I have it at home. Again, about 7 years out of date.
The rain we knew is a thing of the past -
deep-delving, dark, deliberate you would say
browsing on spire and bogland; but today
our sky-blue slates are steaming in the sun,
our yachts tinkling and dancing in the bay
like racehorses. We contemplate at last
shining windows, a future forbidden to no one.
Derek Mahon
"Maybe all one can do is hope to end up with the right regrets."
Arthur Miller