Has anyone ever changed there trany fluids? how often is this supposed to be done? and whats a normal price?

I saw at the jiffy lube place by me that trany fluid change is normaly $30 but since we dont have a dipstick he said he cant to it so i called the dealer and they want $195, and aamco wants $130.

I dont think i ever changed the fluid and i have 75,000 miles on it but i do always change oil every 4,000

adam