I agree that #3 is the most important reason to go do it. To this day, I'm still very proud of what I acheived during my graduate work, and how I was able to really gain knowledge they can't share with you during your BS (due to time constraints). I actually get that "warm fuzzy feeling" every time I think back to graduation day.
As others have alluded to, it's difficult to put a direct dollar value on it. I'd say that's true of most things in life, however.
Having said that, I have been able to sense very important benefits from having an MS degree. For one, I had my pick of where I wanted to work. It seemed like anywhere I chose to interview, I received an offer. Talk about a great feeling. Was it due to the MS degree? If it wasn't, it was due to the added confidence the MS degree gave me. My BS friends, unfortunately, seemed to have a more difficult time in finding a job that suited them. They eventually did, of course, but some of them had to settle for something they weren't excited about, as they described it.
I think it's given me peace of mind too. I feel that if I ever found myself looking for work for reasons beyond my control, it would not be difficult to find in short order. That's important to consider when you have family that depends on you.
Getting back though, the bottom line is that you'd better be interested in the subject matter-very interested. If you're not, it will probably be a long two years. I'm not sure what you'd gain by it either in that case.