'West End' theatres are mainly concentrated around Leicester Square, Covent Garden and Shaftesbury Avenue. These are the commercial venues featuring the mainstream shows and larger and more expensive productions. Ticket prices for these can be high, sometimes as much as £150 for so called premium seats, or even more if you buying from an agency who charge booking fees. However, due to market forces and the sheer size of some of these venues the cheapest tickets can very reasonably priced as well with some tickets retailing for as little as £15 depending on the day of the week (although these seats may not have the best views!)
There are a few West End theatres not actually located in the West End but in Victoria. These are still classed as West End venues due to the sheer size of them with seating capacities of up to 2,000!