At best, I've seen 30 hour (30点) cards go for about 4000. The lowest I've seen is 3200. The optimal price, I would say, is about 3500. Try to sell for 3600 though.
15 hour ones (which don't sell as well) generally go for about 1500-1800.
Try to sell within your guild or social circle, it takes a lot of the danger out of selling cards. If you do have to sell it, try to sell it to people with higher gearscores, don't fall for scams.