There is a viral campaign for the movie Cloverfield. This movie has been getting a lot of press for its hand-held camera style and guerrilla look modeled after "Blair Witch Project", except with a bunch of special effects for the monster (they spent more money - Blair Witch was almost filmed for free).
The economics of Cloverfield will prove bountiful for the producers on a couple of categories 1) they used all unknown actors 2) they cut the movie running-time down to about 90 minutes. How did they cut down the movie running time? They eliminated almost all of the background on the characters and narration type activities (i.e. the scientist in the white coat speculating about where the monster comes from).
A lot of the film critics are irked by the fact that the movie just jumps right into everything and doesn't bother with classic movie elements like back story. However, the writers knew exactly what they were doing - the kids who go to the movie absolutely don't care about any of this and just want a thrill ride, which is exactly what the movie supposedly delivers (I haven't seen it, so can only speculate).
Filming a short movie just has to be cheaper than filming a longer movie. There are fewer shots, less special effects, and less to edit and anguish over. Of course you could have an expensive short movie and a cheap long movie; but the point is, everything else being equal, a shorter movie probably is less money.
The fact that the actors are unknown probably helps the urgency of the movie; I remember watching "The Bourne Identity" while the star (don't want the traffic, you know who I mean) goes through customs and skulks around undetected while all I can think of is "I can't believe no one is recognizing this movie star!" But if you are an unknown and this is a home camera movie, it makes a bit more sense.
From the movie theater OWNER perspectives, shorter movies HAVE to be more profitable than longer movies (unless you get something like Titanic). The owner is paid by the movie; a shorter movie obviously allows you to pack more showings into the day than a longer movie. In addition, the high-profit popcorn and drinks are probably not purchased as a function of time; they are purchased as a function of entering the movie (people don't usually leave mid-movie to fuel up on more snacks; they buy them walking in).
Finally, I recently watched Chinatown, the classic. The movie has a lot of interesting elements and I like the period LA decor. The traditional view of this movie is that the greedy developer ruins everything; but really (to the extent that it is true) it is the story of a visionary that brought water to the valley and extended the residential footprint of LA. It is confusing to me why orange groves (non-native) are "pure and good" but diverting water for residents is "evil and bad". As I note here water is usually drastically mis priced when used for agriculture, and city dwellers will actually use water more effectively than farmers because they are charged a market-based per unit fee.
Since the villain is a businessman he has to be the most vile man around; in this case he has an incestuous relationship with his daughter (when she was 15) resulting in the birth of his granddaughter (and kills his business partner). I was scratching my head thinking why in the heck this is in the movie and then it hits me; the director Roman Polanski was going after underage girls and he just figures if he is doing it, everyone's doing it (see Dan's post on him here). Note that Roman has a cameo in the movie as the guy who puts his knife in Jack's nose and cuts it as a warning to stay away from his investigations; too bad he never received like treatment in return.