Well, if the spline pattern is the same on the alloy & carbon left hand crank arms (I'm not sure that they are the same, but they probably are), then you can certainly buy a COMPLETE new-or-used FSA crank off of eBay for much less than $150 ...
Figure on $50-to-$100, depending on condition (i.e., used-or-new).
What happened to YOUR left crank arm?
FWIW. If you can't locate a crank, I might have an 'extra' 175mm left MegaExo arm that I might be willing to part out & sell to you.