Thanks Contraversial for your tip on searching the web. I was able to find a nice chart with the 1% grade changes and the equivalent pace.
I think the mental toughness point is arguable. I've lived in MN for 8 of the past 11 years and have run many miles in crappy winter weather. Personally I find it as mentally tough to run low 6 minute pace for 8 - 10 miles on a treadmill as it is to run high 6 minute pace outside in sub zero weather. I agree that's it's about the amount of effort. However I definitely think there is an added benefit to having the same effort produce a 6:15 pace inside rather than 6:45 pace outside. To each their own. I should add that I don't do all of my training on a treadmill. Right now, due to the harsh weather, I'm doing about half my mileage inside.
Thanks again for your response. Keep warm.