I've had a quick look and nobody seems to have mentioned this: the Nomadic trait gives you the +15% bonus to immigration but it doesn't give you the -15% resettlement cost. I've tried a few games and moving workers costs 75 with both Nomadic and Corvee System, 75 with just the latter, 100 with neither and 100 with just Nomadic. It's still useful for the +15% but it doesn't quite do what it says on the tin.