take off Instant Upgrade and roll a piece.  Once you get passed I think 8, then it's only a chance to Upgrade. 

Prior to the implementation of Instant, you had to click every single upgrade and watch a bar and watch it fail. or pass. 

Upgrade 1 Level now packs all those fails into the price, which is why you need mythic levels of silver to upgrade. 

Yup, its totally random.  Of course there'll be an average but honestly couldn't even guess what it is. 

Generally speaking, I figure roughly 1.5M to get each piece to 16

I'd say it's more costly than that. It's sometimes 1.5 - 2m to upgrade from level 12 to 16, usually find if it does 3 expensive rolls you then get a cheapo.

We

Probably fairly close.  Free gear swapping should help everyone start to collect a bit more.