I had this same question about my 23' Tahoe. I bought it with 48k miles on it and I don't know anything about the history. Two shops including the dealership quoted me $600 to $800 depending on if I did the filter or not. They both said they do a flush. I then called the local AAMCO transmission shop and they said they do not recommend ever doing a flush, but doing a pan and filter service every 30k miles. They quoted me $399. I'm unsure of what to do. Never been to AAMCO so I don't know what to expect from them.
IMHO, stay away from AAMCO, their business model is to drop the pan and show the customers how much metal is in the pan to sell rebuilds. This may be why they only want to do a filter service?
The issue is ALL automatic transmssion pans and magnets have metal in them, the issue is HOW MUCH metal is too much and how to know what is NORMAL. If the magent looks like a Chia Pet in full sprout, then this is BAD, but slight bit of metal is normal.
Not sure how much fluid would be used during a complete flush and filter change. I would guess between 10-16 Qts depending on towing package etc. But even marked up, ATF at $20 a Qt for 16 Qts you could be looking at around $320 just in fluid. Add a few hours of labor and a filter kit you could be close to $600. But I would shop it aound a bit and look at it this way, even $500-$700 in service 1-3 times is far cheaper than a 10 speed trans overhaul!
You could also do a DIY fluid fluid flush with a few buckets and a radiator cooler fitting and run the engine to exchange the fluid. Best to have a helper to start and stop engine. Search YouTube for some videos on how to do this. Pretty simple, garbage in vs garbage out if you do not suck air the level stays pretty correct.
I have not confirmed 100% but if the truck does not have a transmission dipstick, you must FILL the trans with the engine running, you can fill with engine off, but then you need a helper to start engine to finish topping off level and the fill plug MUST be reinstalled while the engine is running.
