Ooo there's a great video on Minute Food about vanilla extract vs synthetic vanilla. It basically comes down to: if you cook the vanilla, synthetic will taste the exact same, if you never heat up the vanilla it might be worth getting the real stuff.

I assume the same is probably true of most oils, if you use EVOO for salad dressings it might be worth it, but if you're using it to saute you might as well use sunflower oil and save some money.