Why is my transmission fluid and coolant mixing?

I have a 2001 pontiac grand am automatic. It recently started leaking fluid from the transmission. And the low coolant light came on. The car runs normal no tranny slips no over heating nothing like that. It looks like its leaking from the tranny pan so I changed the seal but its still leaking. The fluid coming out is orange and wattery. The people at o riellys said its definietly transmission fluid and I probly have a blown head gasket but why would my car not be overheating at all then? Here is a picture of whats leaking out on the left and fresh tranny fluid on the right

Im taking it to a shop tomorrow but want to prepare myself for what it might be. If it is a blown head gasket how much does that usually cost to fix?