It's free, and happens every hour at night at the Mirage. It's pretty cool to stop and look at the first time you see it, every time after that you kind of just walk by it and if it happens to be going on you might smile, but continue walking (unlike Bellagio Fountains show which will usually make you stop 9/10 to finish the song..unless it's Celine Dion ..ughh). Anyway, it's definitely fun to watch and it doesn't make you wish you could get those minutes of your life back (like Sirens of TI does).
~G