I did Tonto Creek last year with the large black plastic bags as a pack liner, secured with hair ties. It worked alright, but I'm glad I brought extra bags, because my gear would stretch and rip the bags, when unpacking and repacking. I've got a trip coming up to West Clear Creek, and decided to go the drybag route. I shopped around and found a 65 liter military drybag(ILBE system) made by Sealline for $40 shipped, from hankssurplus.com. They sell some places for $90. It's more than enough to line my whole pack and then some. It comes with a purge valve and says it's airtight. I'll let you know how it preforms.
"The only Zen you find on tops of mountains is the Zen you bring there."
— Robert M. Pirsig