I don't know. Take contractors. For many types of heavy lifting, a good contractor who is also a lead dev etc may be justified in asking for 100 dollars an hour or more, simply due to the complexity, their accumulated experience, and just how good and fast and right they are in a tech that's in demand. However, for every person like that, there are n who do not justify a hundred dollars an hour. So, isn't an OK compromise to start at 40 dollars an hour and see if they can do the work of 3 days (or a week or three weeks, or 'unusable') of one of the 'other' devs, more quickly or just better and more reliably, and if so, bump their rate to 100 dollars an hour as they're asking for? Think like Oracle/SAP specialists etc who would do something like build a complete database and web shop, front-end, back-end, everything from scratch for a traditional brick-and-morter store.
To take an opposite example, consider a designer and logo creator. Short of a portfolio, which many designers are not able to provide except to say, look at this, this, and this web site, which they did not work on alone, it is simply hard to know how much they contributed and how much was the art director, other designers, etc. Some designers are independent and bring customers who pay well and basically justify any rate for themselves. (e.g. fifty thousand dollars for a new worldwide corporate logo for a fortune 500 company). Others are worth fifteen dollars an hour. Don't you think a good compromise might be a decent rate (25-40 dollars) for something small, then bump to awesome rate if, well, they're awesome?
Naturally this only works if there is actual trust from both sides. But you would never pay fifty thousand dollars for a logo by someone who only knows microsoft paint and sucks using it -- or even ten thousand or even five thousand -- nor would you pay 100 dollars an hour to a "hadoop engineer" (your requirement) who turns out to not be a hadoop engineer, barely know some html and php, and even with php can't do anything that you want or need. Nor would you pay 80 dollars nor 60 dollars an hour to that person.
So if there is trust that you're really looking for top talent -- and the dev's bullshit detector doesn't go off -- why not do this: "Rather than you doing 10 hours of unpaid phone calls, resumes, interviews to get to 100 dollars an hour -- your next gig -- why don't you do 8 hours of work for me and get to the same place. This applies only if you're worth 100 dollars an hour"
Then if the person is not lying (they're worth 100 an hour, they're an actual hadoop contributor and ju one of the best in the world), and you're not lying (you really want to pay that, for really such a person), it all works out.