Do you mean the base oil itself or the finished blended product?
On the raw base oil end, ChevronTexaco has some technical reports that conclude that Group III is much closer to Group IV than Group II. I don't know what the relative prices of the base oils are for a price/performance analysis.
As far as finished lubes are concerned, prices are all over the place. Group III's like Rotella T Synth, SuperTech and Coastal sell for about 2X the price of Group II's like Chevron Delo and Supreme. Pennzoil, Valvoline, etc Group III's retail for the same as Mobil 1, about 3X the price of the Group II's. Move up to Amsoil or Royal Purple and the premium for PAO is much more.
We try and try, but how to we truly measure performance? Which properties are most important to each of us?
I am about to do a group II vs III experiment that will satisfy my curiosity but might not answer your question. I have been running Delo 15W-40 and changing it every six months, about 5000 miles. I plan on running Syntec 10W-40 for one year in the same car and do UOA. The cost, including filters is a wash, not counting my labor. If the oil does not hold up, I have an answer of no. If it does hold up and I get some other benefit like reduced fuel or oil usage or lower wear metals, the answer is yes. I do not plan on running Group IV or V, because I cannot bring myself to go beyond a year and the price difference would require that.