I purchased mine from Johny Appleseed 1.5 yrs ago - no idea who is the cheapest at the moment.
Re their mount - it is brilliant. From Melbourne I have done Flinders Ranges twice, Oodnaddata track, Birdsville track and , Cordillo Downs, Innamincka with no issues at all.
Highly recommend the 660 and the mount.
2012 Triumph Explorer
2010 XTZ660 Tenere (in the shed)
1996 XR250 & 1994 XL185 (gone)