The lenses are only expensive if you buy them new from Mamiya America Corp. Used, they are cheaper than comprable Canon EF lenses for example. And hellishly cheaper than anything comprable by Leica. Ugh! The price of leica glass is insane.
Plastic - only on the surface. The underside is a single casting, machined to pretty exact tolerances. And as far as inferior to leica build quality... I suppose if build quality means it needs to go to the shop regularly for maintenance, then sure, Leica earns that title.
The 43 is such a spectacular lens. I wish they had released the 43mm aspherical they (Mamiya) designed for the RZ67. Not that it would be better than the M7 lens (not likely possible) but still would have been nice to have that wide a lens on an SLR.