I did a search for a waterproof duffle bags for an upcoming trip and found something that looks fairly decent for a pretty good price and it's also a good size.
We don't travel enough to warrant $180 plus for a full size Tbag.

http://www.waterproofcases.net/duffle60b.html

 

Those look good! You might also consider the Ortlieb dry bags available from Touratech: Ortlieb Bags

They are another low cost alternative and come in different sizes and colors. I use them and they are definitely waterproof.