The Dark Knight is a large budget production.  As such it could afford big name stars, like Christian Bale and the late Heath Ledger.  Also the advertising campaing was large and intricate, using many viral methods as well as the above trailer, which as you can see, displays the film's budget, showing elaborate action scenes and panning and tracking shots of Chicago (as Gotham City), with the camera on a high shot, showing the state of  the art film equipment used on the production.   The viral methods included making a website entitled Ibelieveinharveydent, who as Batman fans would know, is a prominant character in the series.  This site gradually became more defaced looking and "joker-ified" tantalizing audineces further, as no-one had seen the Heath Ledger version of the Joker at this stage.  These methods show that Warner Bros. had sufficent money to spend thousands on promotional materials to whip up internet discussion and exceitment about the film. 

As previously mentioned, Nolan chose Chicago to use as Gotham City, which would have required extra.

Also, Christiphor Nolan (the director) shot certain scenes of The Dark Knight with Imax cameras, which are state of the art cutting edge technology, and would never have been within budget for a smaller film.