What's a self-employment tax?

The way it works here (Australia) is a contractor charges GST (basically a value-added tax) to the employer of 10%. The 10% is standard, so people don't usually include it in discussions of their rates (ie, if he is getting $90/hour, then the company is paying $99/hour).

After that you can handle the money how you like - if you are smart you can setup a company or trust so you only pay the company tax rate on most of your income. At worst you just pay yourself the income, and so you are taxed as though you have the $180K income.