Canon 85 1.8 - $359
Sigma 85 1.4 - $894
Canon 85 1.2 - $2K
Pricewise, these lenses aren't even close to being comparable.
One thing you to consider - Every Canon EOS lens ever made is still compatible with EOS dSLR's. There are Sigma lenses made for EOS film cameras that no longer work with current EOS dSLR's.
I just want to add that there are some Canon lenses that also have compatibility issues. Not the same as the Sigma lenses but Sigma wasn't a serious player at that time. It was only with the boom in consumer DSLR purchases that Sigma started investing a lot more into their own technology. The most recent statements coming from the company get me really excited. All of the new lenses coming out will have every lens tested vs random selections from batches for QC. And the new USB device that you can connect to the lens and make calibration adjustments, control focus speed vs accuracy and focus limiting is incredibly cool. Sigma has a lot going on right now.