http://www.duravent.com/ProductCategory.aspx?c=49
Seen it used on a few boilers. The stuff is not sucure enough for my liking. The clamp system leaves much to be desired.
No idea about cost comparisons but It looks expensive..
Not sure about Duravent but I believe the Centrotherm http://www.centrotherm.us.com/Products/InnoFlue-Single-Wall-Residential.aspx (http://www.centrotherm.us.com/Products/InnoFlue-Single-Wall-Residential.aspx) is less expensive and faster to install than CPVC 636