K-1 wrote...
Are the first seasons just testing the water, to see if a series will prove popular enough for another batch of episodes? Is it a money thing, splitting up the cost over a longer period of time?
I don't have any backing data, but they usually test them out first. Like for example The World God Only Knows, where DVD sales was the main deciding factor for the 2nd season.
Money is also a factor, since buying the rights and the time spot for a full 2 season (24-26 episode) anime, is said to be very costly. Hence why Fate/Zero will be split into 2 seasons.