>At this point, NSF has invested approximately $1.1 billion in construction and upgrades, in operational costs, and in research awards to individual scientists, who study LIGO data to learn more about our universe.

http://www.ligo.caltech.edu/system/media_files/binaries/300/...

But it's important to point out that

1) That $1.1 billion was for two facilities

2) LIGO underwent a huge upgrade over the last decade, making the instruments tremendously more sensitive, and the cost of this upgrade is included in there as well.

So I suspect that building a third facility with the sensitivity of the two existing facilities would be a much more affordable endeavor. The first time around is always the most expensive.