I estimate that supporting IE6 has added more than 30% of the interface development costs to the complex projects I have worked on that use a combination of sophisticated CSS and JS to get the job done. This is a HUGE cost for start-ups and for clients to swallow, once the true cost is explained to them.

Just supporting the way various sub-versions of IE6 inconsistently handle cookies is a nightmare, and in many cases lead to unsolvable issues. This costed a client of mine real $$$$ to discover, and less than 20% of their audience used IE6. Less than 15 people actually complained about the issue.

IE 7 isn't quite as bad, but it still adds a measure of time to front-end development efforts. I would estimate it in the range of 10-20%, depending on how complex the interface is.

On two recent projects, we dropped IE6 support, with virtually no push-back from a general-use audience. IE6 is almost 8 years old. Most people don't keep their cars that long, and can understand that an old crusty browser is not going to perform as well.

The only place where I would be hesitant to drop IE6 support is on marketing sites (where you need to entice someone to a call-to-action within the first 30sec-1min) and on e-commerce sites, where every dollar counts.