10% of the population of the world use IE6, but that is heavily weighted by countries such as China (34%) and South Korea (22%)
If your target audience is English-speaking or European countries, the relevant proportion is 2% to 3% or less.
That's a pretty good level to start to consider whether supporting that small percentage is actually making you more money than you would if you didn't support their browser.
Does anyone know why Asia has such a high level of IE6 usage? I don't think it has to do with the computers being old, because IE6 usage is relatively high even in developed places like Japan, Hong Kong, and Singapore.
A major reason in South Korea is that in the late 90s, the government mandated the use of ActiveX for online banking. This was only recently repealed, and the Korean government is now encouraging people to move on.
The government mandate encouraged many websites to also use ActiveX. South Korea is probably the only country where ActiveX is still in use. Internet Explorer has a browser share of 90-95% in Korea. IE6 is about 15-20% of this.
A clarification on Korea's ActiveX problem: Korea mandated 128 bit encryption via the use of an ActiveX control or plugin, but when Netscape bit the dust, there was no reason to update that plugin anymore.
If your target audience is English-speaking or European countries, the relevant proportion is 2% to 3% or less.
That's a pretty good level to start to consider whether supporting that small percentage is actually making you more money than you would if you didn't support their browser.