We already know Canon can make lenses with IS for $130 so why the extra cost?
Glass has to fit the full-frame view, not APS-C, which is a big factor in the size of the glass, etc. Cheapest full-frame IS lens from Canon prior to their primes was what, 70-300 at $650ish?
Also, any lens Canon releases from here on out has to hold up to 40+ MP on full-frame, which is more demanding on the design. Notice that all the new primes are much sharper than their predecessor and generally sharp across the frame.
But even if I was a video person, I might go for the Sigma anyway. The extra stop can be key for darker scenes, and either a rig (if you're walking around) or a tripod negates the need for IS.