Well, in three years full time instructing, my average was around £15k gross, and asking my co-workers, they were all saying about the same.
The way you live on this is living at the bare minimum, in a house share, and living on credit cards and over draughts.
For most it is only short term, a year is quite common, I instructed for three before I was forced to move onto an airline position for financial reasons. If I could earn what I do now as a full time instructor, then I would. I still instruct part time to keep the rating current.
Once you have some decent experience, and if you're in the right place in the right time with the right contacts, you could easily land a well paid instructor job as CFI, or a contractor for the military. It's really all about who you know at that level, but I'd say you'd have to instruct for at least a year earning naff all to build the experience and get unrestricted (which makes a massive difference to what you can do.)