You may be able to get the Nest Power Connect for free from Google.

I got my nest from this promotion last week (came pretty quick), only to find out this version needs a common wire. I have two Nest E thermostats for other zones that work fine already so I didn't think much about it. I coughed up another $40 for the Nest Power Connect and a trim kit. I got it all hooked up and it works ok, but I still get some occasional errors about unstable power. Then today I got an email from Google offering a free power connect to help with the power issue. So maybe you can skip a step and get one free from Google.