I think most of the people working for those sorts of salaries on the expat packages are working for foreign companies and have been sent to China not necessarily by choice. There's a category of people who basically move from country to country whereever their company sends them. They get high salaries because living in another country is considered a "hardship." They also get perks like paid tuition at international schools for their kids, housing allowances which allow them to live in ritzy compounds, insurance for Western standard hospitals, paid drivers and ayis, relocation allowances, all sorts of stuff.
Eventhe folks not technically on expat packages have jobs that are not really for your average foreigner in China. If you have really nice (corporate) qualifications you should be able to land a good "local hire" job at a foreign company and maybe make somewhere around 20-30k-ish. I had a friend working for Siemens in Shanghai and she was making about 20k, and if she had stuck with it for a few years she probably could have worked her way up to a bit higher salary. She went back to the States and then was hired from the States for a job in Guangzhou and is making more than she was at Siemens because she is now a "foreign hire." A big part of how much you make is based on whether or not you were in China to begin with when you were hired, which is kind of unfair I think, but that's how it goes.
To be honest I don't think most people teaching English in China are capable of making 30k a month, so foolish or not, there's not much that can be done about it. The only way you can make that teaching is if you're certified to teach international school, and if we were all certified to teach international school I'm sure most of us would be doing that. Not everyone can get a corporate job either, most people don't have the background to even get an interview, much less a job ... not to mention the desire to work in the corporate world in the first place! However, the idea that you can't live on less than that in Shanghai is ridiculous. I live just fine in Beijing on half of that, and that's with 2 kids to support. I suppose if I was hitting fancy clubs every night or visiting a lot of massage parlours (or their equivalent for us ladies, hehe) then maybe my salary wouldn't be enough? But my family is hardly roughing it so I think those people are just full of it.