I value main/con/luck/secondary stats at 10/9/5/1. I'm not sure why. I think I read it somewhere, and it seems to work for me.
So a 5 stat item at 100 per would be rated as 100 + 90 + 50 + 10 + 10, or 260. A 3stat item would be rated 100 + 90 + 50 = 240.
I'm not really sure how to derive the optimal formula, or if the optimal formula differs for different classes. What you'd really like to know is what distribution of spending on stat points gives the best chance of winning battles.
