Someone selling a Will Piatt tuned 2x tube on the yellow forum now. Of course it's more than the non-tuned new guns. http://www.network54.com/Forum/79574/message/1418183680/Tuned+909+Double+tube
There are deals out there if you're patient. I got mine with a LDC for $500 shipped in a hard case. It looks brand new and the seller said he shot less than 200 rounds through it. Most instances I would have just considered it a sales pitch but in seeing the gun I believe him.
Sportsman Depot has the twin tubes in stock for $654 but PA also have the twin tubes. I guess you're looking for the 909S for more power?