I guess the 5D3's CA lens correction thing really helps the 85m f1.8 then…
i personally don't use the 85mm much so i couldn't justufy getting the Sigma when i got the 1.8 at about USD300 used and even at 1.8 (which is use very often when i use the lens), i don't get the purple or green tint..I wouldn't say it's a super sharp lens but it's very worth the money and unless you are going to use 85mm alot..i don't see how spending so much more for the Sigma or the 1.2 makes monetary sense
I've also heard that the 5D III does correct for a lot in the camera. As far as monetary sense or not (this is not my occupation, but I do some small jobs now and then) I think for me all camera expenses are justified for my utter joy of taking pictures. I was shooting a wedding (after the paid photographer had taken the official shots during the ceremony), and I did it as a favour to a friend (I was also a guest). I had brought my 135 F2.0, 70-200 F2.8 II, 16-35 F2.8 II, 24-70 F2.8, and finally my 50 F1.4. Everything was fine, but I sensed that the 135 was too long for many of the shots, and afterwards I felt that 85 would be a good range lens while moving between people.