A few years ago i bought an M3 for quite cheap. After few months of use it showed signs of needing a CLA. I sent it to Leica with request for CLA and special request to make the viewfinder/rangefinder as good as possible and to ignore any cosmetic defects. Price included some new parts like filmplate, shutter curtains and bayonet, the parts cost about 250€ and the repairs about 550€. Price might seem ridiculous, but i´ve been happy with my M3 that functions like a new one, and looks like it´s seen a few thousand rolls of film (which i´m sure is true...).
As far as i know Leica doesn´t do light or heavy CLA´s. It´s always the same, and the parts that are in need of replacing are replaced. They do give options too, but that´s for some extras, like a different film advance lever or something. They also don´t give estimates for repair costs without seeing the camera first. They check it then send you an estimate about the costs and if you think the cost is too much they send the camera back to you.
Niko