Back when the Mirage was rumored to be for sale there was a lot of discussion about how much the sale price would be, and how it would compare to other similar properties, say Cosmopolitan for example.
With Phil Ruffin's $1.3 billion offer to buy the Mirage rejected, we can determine that it would be worth more than that.
Cosmopolitan sold for $1.73 billion back in 2014. Although MGM Resorts has repeatedly stated that Mirage is not for sale, I would think they would need an offer of around that figure to raise their eyebrows.
If I had to place a value on Mirage I would place it right around that $1.73 billion mark. Now this is an unscientific estimate. I haven't dug into the financials like I probably should have, but I think it would be a fair price for the Mirage if it were ever to be on the market.