NX is bound to suck up heaps of bandwidth even over ssh-gzipConsider:
"NX requires an average of 20Kbps (kilo-bits per-second) to let users run a remote Linux session confortably (sic) ... NX can work on slower links, like 9.6 Kbps GSM connections, but in such network conditions users can experience a noticeable lag."[1]
The HNer asking the question has given us the baseline of 7 GB, though it's not exactly clear how much he uses per month, I'm going to use this as a maximum and consider anything less than 7 GB a gain in efficiency.
7 gigabyte = 58720256 kilobit,
58720256 kilobit / 20 kilobit per second = 2936012 seconds,
2936012 seconds / 60 seconds = 48933 minutes,
48933 minutes / 60 minutes = 815 hours,
815 hours / 24 hours = 33 days
So, one could browse the internet for 33 entire days before using more than 7 GB. It is, of course, impossible to browse for 33 days in a one month span, so we've gained efficiency even assuming one browses the internet every second of every day for a month.
Let's say our theoretical user is on the internet 12 hours a day, that means it would take said user 66 days to use 7 GB of bandwidth. We've gained 102% efficiency!
I think it's more likely that our theoretical user is actively using a web browser less than 4 hours per day, so if our theoretical friend uses a local X server for all his non-internet needs and strictly uses a remote X server only when he needs to view a web page, he could go more than 198 days (!) on 7 GB.
But I'm not finished yet! If our theoretical user doesn't mind "noticeable lag", he could further throttle his connection speed to 9.6 Kbps, which works out to 76 days of internet time. If he actively uses the internet less than 4 hours per day, he'll be able to go more than 456 days on 7 GB! That's 1420% more efficient than the original scenario.
Best case: > 1420% more efficient!
Did I mention our theoretical user doesn't have to give up his beloved GUI?
[1] http://www.nomachine.com/ar/view.php?ar_id=AR12B00119