You might also like to run metrics as to how those same IE6 users evangelize your service. They might be really good at word of mouth, or at recommending other enterprise-level customers in via the golf course.
Or - more realistically - you could actually consider 10% of your customers a large, valuable chunk whatever the ROI and accommodate them.
I think this is talking about immediate ROI, that is customers that actively purchase products, and indirect ROI, or customers that serve some other valuable purpose.
As most sites follow some kind of 80/20 rule, be careful to dismiss a small percentage of users if they fall in the critical 20%.
Then again, if you have data that shows IE6 users are just parasites, get rid of them.
Or - more realistically - you could actually consider 10% of your customers a large, valuable chunk whatever the ROI and accommodate them.
Your choice.