Depends on the tools you use, etc. It very well may double the time required (as IE 6 support requires tons of testing of all the features, and many software project have huge portions of the projects being testing, not development).
Additionally, you tend to need more experienced engineers, IE6 support is aged out of the current crop of grads.
I think the second legacy browser site is a great solution. Additionally, I think if you're saying "has little impact on the final product", you're not working with clients who open up psds and count pixels on you. Some people are.
Trying to build a responsive, dynamic webapp, that is identical in IE6 to a modern browser can easily add a lot of time. Just as an example, producing all the graphics for rounded corners, rather than being able to do them in CSS.
The thing about IE6/7 support is that it's a complete known quantity at this point. The problems & workarounds are well documented. There's tons of experienced HTML coders out there that know IE backwards and forwards. CSS frameworks like OOCSS include built-in workarounds.
So if IE support doubles the cost, it must either be something very complex, or a real nickle-and-dime project. (Either that or the webdev doesn't really what they're doing.)
That said, I'm happy to drop IE 6 where not required by the client - 8 may not have html5 or CSS 3 features, but at least it's relatively bug free.
Having said that, fields with a more even gender ratio have had to deal with this for a while - what do they do?
Just because a book said it was preferred in 1848 to use he when sex is indeterminate (then went on to use both singular he and singular they) doesn't mean that's true today.
The language always used both. Why stop today? Might feel a bit odd at first if you're not used to it, but it's not a bit different than how singular "you" is used ("You are coming today?" uses the plural form of "be").
The book itself used both forms! Shows you that English teachers may have been overly strict with you in the past.
Some more on the topic:
Anyway, "They" has been undergoing this process for a few centuries now, and I concur that it's time to embrace it.
I don't really think it's an evolution, as much as the mid1800's tendency to over classify and striate things going away.
It is one of those niggles that irks my pedantic self though; particularly the politically correct change to using 'she' instead of 'he' (instead of the perfectly neutral 'they', and even 'one').
I've always used "he/she". But that's not quite accurate (or elegant) either.
While supporting IE6 is onerous extra work, it's still work that can be done and has little impact on the final product.
Take the other position, that 'Not supporting IE6 will mean I'll charge you 70% of my original quote, only x% of people use it in your target market' or better add it as an extra in the quote in the first place as the article suggests.
Then it's up to them, it's a risk assessment. But be up front in your quotes with it, not an after the quote is won 'I don't know how to do it!'.
The article's incredibly OTT on how much adding IE6 support costs, but I certainly would never go down the route of lying to a customer about it. If he charges double it's probably because he hates doing it and fair game to him.