In fairness I edited my previous post (PS & PPS) after patrickt posted.
Food production, like manufacturing widgets, prices have came down because of improvements that reduce costs. We have better fertilizers, insecticides, no till planting, better machinery, etc. A farmer can plow in an hour what would have taken weeks with a horse. Of course the prices have come down! Your comparing grapes to watermelons.
Energy has not changed. A btu is a btu and always will be. Yes the cost to recover energy changes, and supply verses demand impacts price. I expect we can produce a barrel of
oil cheaper today than we could in 1900, too. However I cannot think of any one thing that is more closely tied to the economy and more stable in terms of value than energy.
Now if you think the governments inflation index is a better number, fine use it. Find the year minimum wage was $1.50 and run the numbers. I expect you will get similar numbers, but it would be very interesting to know.