The only wheels I know of that are lighter than Monocoque would consume too
much of your annual salary. Similar in weight & cost to Monocoque is
Litespeed (formerly Taylor wheels) and their wheel parts are made by Bogart;
a sister company to Monocoque.
I bought a set of 3-pc Monocoques w/forged centers in 1996, originally for an
M3 and now used on my Z3 coupe. The 17 x 7.5's are 13.5# and the 17 x 8.5's
are 14#. The outer rim sections are typically available in 3 thicknesses;
0.120, 0.165, & 0.210 (if my memory serves me right). These are true custom
racing wheels. You will need to supply all the proper dimension
specifications for your application.