Curious to this as well. Wanted to get it at the dealership, but my previous truck got totaled so I needed a car now. Plus they were talking into the couple of grands and I didnt feel that it was necessary, I liked the way it looked enough to buy it. But I would still be curious as to what it would cost.
I had one installed on a then new 2003 Century from the Buick Dealer - they used someone near by that was authorized by GM to do the work and give it a lifetime warranty. The work and install was okay plus they used the correct factory drop-down headliner inside - full tilt and hidden type with tinted glass and inside sliding sunshade.
Some notes to consider though - the outside metal roof was cut and the trim ring was on top the roof surface - no mandrel bending around the edges, just the trim ring hiding the cutout. The sides of our roofs are very thin and will bend with the slightest elbow pressure - they will put little dings all the way down the drip rail. The way our headliner comes out is though the front windshield - it has to be removed and reinstalled, chance for that to break plus extra cost involved.
In 2003 the sunroof was $1000 more so I'm sure it has gone up since then.