The fan-site Darth Hater conducted a survey of 2,500 respondents regarding their interaction with the Cartel Market. It seems subscribers have been spending real money in addition to their sub fee. Many of them spending $50+ in the store. The number one purchase has been the Cartel Packs that contain random items followed by legacy unlocks. Less than half of the respondents have sold their Cartel Market purchases on the GTN.
Of course these results could be skewed since it was taken from a fan-site. Also, I don't know if I should be surprised by how much money subscribers are spending. Obviously the more people like a game the more money they are likely to spend on it. One just needs to look at the success of WoW's cash shop. I think the bigger question here is; What does the success of the Cartel Market mean for the future of SWTOR?