transmission leak

I noticed some fluid seeping from the rear transmission seal, it is around the rear of the transmission just behind the transmission fluid pan. It appears to be a very mild leak and does not drip. It appears to be worse with the engine running but improves after the engine is off. My question is should I be concerned about this? Do I need to have the transmission resealed and about how much should I expect to spend on this?

[img:12bbd37696]http://www.2carpros.com/forum/automotive_pictures/574413_IMG_0374_1.jpg[/img:12bbd37696]