Real world FE difference is about 10% for the same driver and conditions. So if you were spending $200 a month for a V6, it would cost $220 for a V8 of similar vintage. Today's V8s are getting far better than the 12mpg lifetime average I got in my 2003 F-150 5.4L. High teens average and low 20's highway! That's the same as the minivans we have had!
Look at these histograms:
Taco V6 17-20mpg
Frontier V6Frontier V6 16-20mpg
There isn't as much data on GM's 2014 models yet, but the average is 17.1mpg for Silverado V8. Ram is 16.5mpg.