I had my fluid replaced at 74k miles when i did my converter, find a reputable trans shop (assuming you can't do this yourself), that know about the 6l80e trans, and ask them to check/change the fluid for you. It would be cheaper than what the stealer will charge you, unless the dealer has a coupon that would make the price attractive.
There are also more transmissions out there that would requiere the same filling/check procedure, so trans shops should know what do, i gave my trans shop the details about the temp just in case, but they knew the trans either way, so it was more for my own piece of mind to know they had the correct procedure at hand. Again, make sure they know the trans, you'll be surprise how many shops have no idea about the trans when you ask them specifically about it. Dealers would also have this info at hand straight from GM.
Some people reuse the filter, but you can get the filter, and the gasket for cheap online.
I just bought the Camaro pan, filter, gasket from Pace, since i was doing the verter, and the parts were very cheap (about $80 shipped to my door), , so i figure the stock trans gasket and filter should be cheaper than the Camaro pan with all the parts, although not sure about that.
My trans fluid was still decent looking at 74k miles, the guys at the shop saved the old fluid to show me, and they said it was in fairly good shape compared to other fluids from other car brands with about the same mileage. I wouldn't have had any issues letting my trans go all the way to the 100k miles mark from GM, so that was a good thing.
I'll use the same shop again the next time i need to replace my trans fluid, and i'll probably get at least a new gasket, assuming the filter can be reused at least once.