The Financial Times reports that seeking to spur demand, Apple is pushing the networks to lower the prices of TV Shows on iTunes (registration mat be required at that link) effectively by half, lowering the price from $1.99 to $1.00. The networks are resisting this, fearing a repeat of what happened with the music industry -- a huge increase in downloads, but at the expense of not selling nearly as many CDs.
With increased penetration of DVRs and the likes of Netflix already putting a dent into TV show DVD sales, it's hard to fault the networks and studios for resisting. Plus, Apple is still going to want their third. At $1, that effectively drops the cost of most TV shows down to $22 a season of which, the studios would receive about $14.50. Still, I could see some of the broadcast network shows coming around to the price drop, with less chances of shows on the cable networks getting that cheap anytime soon, and even less chance still of shows from premium networks dropping that low.
HBO usually doesn't even make its shows available on iTunes until after the seasons are already over. For example, you can get the first three seasons of HBO'son iTunes, but episodes for the fourth season are not yet available). While the DVD market might be dented, revenue for the DVD sales of is estimated at over $61 million. This explains why season two still isn't available on iTunes even though it's season has been finished for month. You can pretty much count on it not being available until the second season DVD is released.