The most basic thing to say* about why theater tickets are expensive to buy is that theater is expensive to make. Economist William Baumol famously observed that it's really hard to make a string quartet more efficient, because it's not feasible to replace the live violinist with a robot or with a cheaper Chinese violinist Skyping in from Shanghai. Similarly, you can't perform "A Long Day's Journey Into Night" with fewer than four principal actors. You also can't make it without a director, or a costume team, or a set designer, and so on. Prices are people, and theater is labor-intensive work, and that makes a night at the theater necessarily an expensive thing to consume.
But it doesn't explain why average paid admission has increased by a fifth since before the recession to $93, according to new statistics from the Broadway League -- nor why non-musical ticket prices grew by about 30%. Here the answer comes from the demand side. Broadway plays and special events are limited-time-only star-studded events that draw lots of rich people. And producers have gotten wise to the idea that rich people are extremely price insensitive when it comes to seeing celebrities on the Great White Way, especially if there's only a short window. The message rich people appear to be sending is: If you charge it, we will pay it.