I don't understand why cinemas are charging more for 3D films, it doesn't cost more to show them, in fact it should probably cost less if they're after upgrading from decades old projectors. From a distributors stand point the film costs a lot less to distribute and the films can now be stored on solid state cartridges instead of on thousands of Euros worth of 35mm print.
Sure the projectors are expensive to install but it's not like they need to repay the cost on them straight away as they'll be operating for the next decade or two. As far as I've seen the digital projectors are very easy to operate and maintain meaning you could probably cut down on staff costs as a result of them as well.
The only added cost I can see is the glasses but if people continually re-use their glasses then that charge should be taken off the ticket price.
What's more insane is that most of the money handed over at the box office goes back to the distributors meaning the cinema gets very little of it, certainly not enough to continue running on. However, the main problem here is distributors as if cinemas don't make enough money on films then they won't get the latest greatest releases which would effectively kill the film.
The main reason cinemas are charging extra for 3D though is greed, it's a limited service which isn't quite widespread yet so people will travel far to go see spectacles such as Avatar and, more importantly, it's not feasible for most people to set up a 3D experience at home.The only place people can see 3D is in certain cinemas and they know it.