If you can change the transmission fluid, filter, and transmission pan gasket yourself then it can be as cheap as $100.00 to complete the transmission fluid change.
If you have a shop do the job it could run from $200.00 to $300.00 and sometimes you'll need a shop to perform the transmission change because it's much easier for them to do the job when they can put your car or other vehicle on a vehicle lift and drop the transmission pan and filter from above than it is to try doing the job in your driveway or other part of the ground.