Not just London -- I used those examples because the website is in English and easily recognized.
Sometimes it's a lot cheaper -- I saw an opera in Vienna for 3EUR, and a youth orchestra (presumably a skilled one, given the venue) playing in Paris tomorrow is free!
The same for the London Philharmonic Orchestra, which is a registered charity.
At least in Europe, the cheapest tickets being around that price is typical.
http://lso.co.uk/whats-on/2016-17-season.html
https://www.lpo.org.uk/