In the case of the Led Zeppelin concert, I'm convinced that the ticket prices were grossly underpriced based on how many people wanted to purchase tickets.
The Rolling Stones performed at the "Joint" a few years ago. The Joint is the concert hall within the Hard Rock Casino, Las Vegas. Ticket prices started at $500 and were sold within minutes.
Some bands will play for private audiences and their fee for doing so isn't cheap.
So the answer to your question is that concert tickets are expensive because their is a willing market that will pay the price.
