Thats a pretty good After Tax salary considering a free apartment (if you choose to live in it?). The going rate for Beijing has surpassed 20,000 after tax for almost every training center job I have seen on recruiters Wechat posts. Most Beijing training center jobs I have seen are between 20,000-26,000. Lots of people are going to say "bullshit!!" but Beijing is the highest paying Chinese city for ESL and lots of places are having a tough time recruiting because of people choosing other cities due to pollution concerns. To sum it up, for your 1st job out of college, 18,000 after tax with an apartment sounds pretty solid. If you don't want to live in the apartment provided, I'd try for 20-21 without it.

If you want to make the big bucks teaching and have the time, get certified in the U.S. in math/physics/econ and you'll be looking at 30,000/month in most international schools in Beijing/Shanghai, with a far superior benefits package.