Well TBH how we do it is we will code in the majority of browsers, IE 7,8 Firefox and now Chrome is over 10% Chrome. Safari, which tends not to be an issue as we now code for Chrome, Opera and IE6 are classed as minority browsers and as a result it's Â£45/hour to code for it.
What you find is TBH people tend not to care or if they do we make even more of a profit for the trouble. Either way for our workload and since I have taken on my job I have learnt how much people will pay for simple things and that its not worth adding an hour on to your development time for the same price.
End of the day, especially with IE6 if more people did this more people would use more advanced browsers and Opera would get round to fixing their bugs, but our clients don't care TBH 90% of our business, if we don't turn it away, tends to be repeat proving that.
Cost your project with good ie6 support included, but offer the client a discount for passing up full ie6 support. This way you are not hitting them with an extra charge, but offering the option of a discount instead - turn a negative into a positive.
Also, it's not as clear cut as ie6 being supported not not - agree on the level of ie6 support: full rendering fidelity being the most expensive and time consuming option, while just making sure the site works in ie6 (but may not look fully as intended) is more realistic.
Jumping through flaming hoops to make an elaborate design look just as intended in ie6 is a waste of time and money and can compromise your markup which is bad for performance, accessibility, mobile support and standards compliance. It's not so easy to explain this to clients, but offering a financial incentive can help.
I would say that the majoirty of users of IE6 are instituions but I would also say thats beggining to change in a big way. A couple of our big clients are now running IE8 accross all their systems.
And as far as I know since we've stopped supporting it, before my time with the company, but we haven't had one client come back even asking us how much to code for it yet alone asking us to support it and we give them all browser useage data for thier own site(s), and loads of other details most will know what I mean.
I honestly think that now 8's out and 9 on the horozion, and that all new windows machines come with IE8 that a lot of the big companies are starting to take the plunge and say maybe nows the time, especally as more and more people are dropping support for it and thus they need to upgrade to keep on top of productivity. Where as before support was still widespread so producity wasn't effected.