Personal anecdote: I just finished an engineering MS from NC State via their Engineering Online ( http://engineeringonline.ncsu.edu) program. The per-3hr-course cost for EOL courses is roughly $900. For the on campus version of the same thing, it's about $2200. For a 27 credit hour program, that came out to about $8k, which I thought was entirely reasonable given I [as a student local to the school] always had the option of taking advantage of campus facilities/options (libraries/labs, office hours, study groups, etc), too. An added perk was -- as the GT FAQ notes -- the ability to work at my leisure. With a full time job and two toddlers, it took me almost four years to complete the program, but I did and it was a fantastic experience.

<edit> To address a question below about GT's program, NC State's is practically equivalent to their campus-based stuff. You apply, and are admitted, following exactly the same process as everyone else. After admission, you follow the same rules and processes as everyone else. The one exception I've found -- besides making it easier to study part time and draw out the total duration -- is that professors are usually much more lenient with deadlines for their online students. They know a large percentage of online students are doing it that way because they're working full time (or are deployed military, or any of a plethora of other situations), and they offer a lot of leeway. This helped me & my peers quite a lot.