"In comparison to the standard 2.39:1 Technicolor/Techniscope presentation most Spaghetti Westerns were filmed in, The Great Silence was filmed in the cheaper widescreen (1.66:1) Eastmancolor format "
First: 2.39:1 is a mistake, you mean 2,35:1
I doubt that 1,66:1 is cheaper than a usual 2,35:1 widescreen format, cause the used film stock is exactly the same 35 mm film stock, which actually has an 1,37:1 aspect ratio. And I see no reason why the postproduction of an anamorphic shot film should be more expensive than the non-anamorphic counterpart. But maybe others know more.
And then it seems that TGS was unlike Django intendend for an 1,85:1 aspect ratio, as hard formatted 1,85:1 35 mm copies exist. And newer DVDs are in this aspect ratio, and the trailer on the old DVDS was also in 1,85:1.
In fact it is Techniscope which is cheaper than other formats cause it needs only half of the film stock in the shooting process.