Well, I'm assuming that you have the Tiptronic, because even the dealer couldn't take $7 Mobil 1 and upcharge it six-fold.
For the manual, there are lots of choices for gear lube, many 'better' than oem for around $12/qt. such as RedLine, Royal Purple, etc.
But, for the Tip S, there's only one fluid you s/b using - ESSO AUTOMATIC TRANSMISSION ATF LT 71141. Use anything else, and you're in for trouble.
Porsche only sells this in 20 liter containers and a complete tranny flush and fill only requires 7-9 liters. But, once they open the container, you're on the hook for the whole thing.
BMW, Jaguar and some others use the same tranny as the Tip S and you can find the fluid in single liter containers at Bavarian Autosport -
www.bavauto.com currently sells it for $16.95/ltr. -
Esso LT 71141
Buying 9 ltrs. would also qualify you for free shipping and this would save you $25/ltr. over your dealer quote. If you brought the fluid to the dealer, you'd save a ton of cash.