It depends on where you're teaching and your own personal living habits.

Locations:

You can make some pretty good money in Japan teaching ESL, but you can spend some pretty good money there as well. The cost of living is rather high in Japan. In South Korea, you'll do better. Teaching jobs in SK pay somewhat less than those in Japan on average, but the COL is less, at least in most places. This will allow you to save some cash. I teach in Thailand, and the pay here ranges from acceptable to miserable. There are some great-paying jobs in Thailand, but you'll probably not be in a position to grab one of those, at least not out of the blue. It'll take a good effort and some patience to land a job making more than about 1200 USD per month in the land of Thai. And you'd better have a master's for most of those. As for China, I'm sure it mirrors Thailand. Western Europe pays better, but you must either have an EU passport or have a very close relative who is an EU citizen (in case you aren't from the EU).

Lifestyles:

Many a recruiter will try to rally you with the old battlecry "your money will go further in xxxxx." Maybe it will and maybe it won't. It's the same story as back home. You have to watch your spending anywhere you end up no matter how far your money goes in your chosen host country. If you live simply, you'll be able to save at least some of what you make. This may nor may not be much, depending on where you teach, but in every case if you want to save, you'll have to live like a hermit most of the time.

Also, contrary to what many a recruiter will tell you, things aren't necessarily cheaper in these places. Usually, you'll find necessities in your new host country are somewhat cheaper IF they are produced there. Imported items will cost just as much, and sometimes even MORE, than they will cost back in the US at say, a Wal-Mart. Believe it. And many times you'll want to buy an imported item, as the 'home grown' version, assuming there even IS a home-grown version, has absolutely atrocious quality and will need to be replaced in no time if you buy it because it's 'cheaper'. In most cases, it WILL be, and you'll soon find out just how cheap if you buy those inferior items. I once bought a 'cheap' portable CD player here and thought I was getting a good deal. It wasn't a 'great' deal, but I spent a little less on it than I would have at Radio Shack. Shoulda gone to Radio Shack. My 50-dollar CD player lasted 3 hours.

In conclusion, I'll say that the salary ESL teachers make isn't necessarily on the level of 'burger-flipper'. If it were, there would be a LOT fewer foreigners teaching overseas, especially in South Korea.

There is also more to ESL than what you think. At many of these jobs, you'll get free housing, free food, free visas, and even air-far reimbursed (espcially in South Korea), so it has its perks if you demand them. There are other benefits, as well. I've been teaching nearly two years in Thailand. In that time I've not paid for rent, car payments, car insurance, car repair, not one gallon of 3-dollar-a-gallon gasoline, or even health insurance. This past week I went to the dentist and had two fillings put in. This would have run me over 600 bucks in the USA. Here it cost me 15 bucks and the results are just as good as what I would have gotten back home.

Don't compare ESL to a job at Burger King. It's not really comparable. If it were, many of us wouldn't be doing it. I know I wouldn't be.