Have you see Lucky9876coins' post? I would consider him an expert.
http://boardingarea.com/blogs/onemil...r-point-worth/
I think his airline numbers are a little high, but the comparative rank is about right. He doesn't include WN, though. I value each Southwest credit at US$15, which would be about $240-250 for an award. I would also be roughly ambivalent between taking 1 WN credit and 1500 carrier miles (not Delta), so on his scale 1 WN credit is 2000-2400.