Average Salary: $93,340
Expected Lifetime Earnings: $3,897,000
It's "eih" at best. You don't become a flight instructor to make a lot of money.
Backtrack to pilots: There was a time in this country when pilots really could make a lot of money—the United Airlines union was famously adept at negotiating awesome pay for its pilots. Much of that pay came in the form of stock options or grants in ownership of UAL, Corp. (the parent company of United Airlines). The stock did well for a while and some pilots were multi-millionaires in stock. But then the company fell on hard times largely because of its bloated cost structure and onerous union contracts which didn't allow it to adjust to fast-changing market conditions. The company eventually "re-capped" itself as it filed for bankruptcy and those multi-millionaire pilots lost all of their stock.
But if you are an active CFI with, say, 30 hours of billable $80/hour client time, that’s $2,400 a week times 40 or so weeks a year, and that gets you just shy of $100k a year. (Why only 40 weeks? Wx. When it's bad, you just don't fly.)
These numbers are for the higher end flight instructors in urban areas. There are tons and tons (and tons) of mediocre flight instructors who are generally bitter and angry about their lot in life. They have oodles of hours to sit around the shop and stay totally caught up on post-The Bachelor gossip.
For the lower end, you can expect something closer to 30 bucks or so an hour and to only work 20 or so hours a week. Using advanced calculus, that gets you $600 a week and in a big city, that'll barely pay for your parking slot in a garage. So if you really do care about the dough, think thrice before entering this gig or you could end up in a place in life where you uh... have too much time on your hands.
There are other ways to make money other than just taking a salary. Pilots who service private jets often get into the leasing and maintenance businesses. Or they fly themselves—and the going rate for a pilot with CFI ("high end") credentials is about $600 a day plus expenses. Not massive wealth but not a bad gig. And oh, the view is nice.