I bumped into this interesting little website today:
https://www.openhub.net/p/flightgear
According to the site (using the COCOMO statistical model to estimate work time), since the first commit by one Curt L. Olson in 1997, there has been 208 person years of work, worth over $11m dollars based on average salaries.
All for free. To all those who have given up your time and energies to make this sim great, you're the best! Here's to the next 200 years being just as amazing.
(FGData apparently involved 1500 years work, valued at $85m. You could buy a plane for that!)