As said over and over again here, engineers and scientists don't pay for grad school. You'll get funded, and most likely also have a stipend to live off of. I even know a girl in PolSci who gets paid for. She has to do TA work for three of her five years, and the alternate years are on fellowship.
I'd say that an MS in engineering isn't worth a whole lot. Those who I know who left school with one were the ones who burned out or were asked to leave when they weren't making progress. Now that I'm in work I know a lot of engineers who do an MS on the company's bill. They get a bit of a pay raise, but they don't really get much out of the MS program (at UCLA!) that leads to them knowing more or anything like that.
A serious PhD would be very, very difficult while working... at least from my EE PhD experience. There was no summer, and there really wasn't a day or night. We just worked and thought and worked. It was fun. Lots of good science got done, and I got a lot out of it. I walked into my current job with a lot of resentment from the older engineers because they didn't understand how someone "fresh out of school" could be put ahead of them. I did a lot of work in silicon fabs during the summers when I was in undergrad, and I had a very, very hands-on/experimental-based grad school experience (rather than a theoretical one). In less than six months, it's well established at my work place that I am the one to go to with questions.
I've overqualified myself for many jobs by getting this PhD, but those are the job I was avoiding. That was the point of getting the PhD. I can now have one of these few jobs that requires a comprehensive understanding and massive experience.
I mentioned the PhD work was a lot of fun too, right?