These are my favorite. I see too many transmissions fail just because of simple neglect. You want to make sure you flush your fluid before it gets dirty, not when it is dirty! When its dirty is too late. I suggest flushing or doing a drain and fill every 30k and doing the filter(if your vehicle has one) every 90k. Its the cheapest insurance you can do to keep your transmission working in good order.
This includes draining and filling the system twice with up to 2 gallons of transmission fluid. The next we have to offer is an actual transmission flush with the machine. This flushes 4 gallons of fluid. This service costs $134. Some vehicles will actual have a filter inside of the transmission pan that can be serviced. To replace this filter the cost would be around $150. If you pan is also leaking this would actually fix that problem at the same time. If you wanted to combine the filter with an actual flush the total would be $194. Though I would only recommend this service for neglected transmission or every 90k miles.