More Words, Deeper Hole

Currently it is sustainable, though more expensive, to grow crops like mustard and soybean for oil to be converted into biodiesel. You get out roughly 200% of the energy you put into the farming and the conversion. But this is expensive relative to drilling for oil and refining it.

So if there were a doubling of yields on American farmland it would indeed mean a reduction in prices, but not all the way down. At current cost levels for oil prices would fall 17% and then stabilize at the level where it becomes practical to replace standard petroleum diesel with bio diesel. If world demand for energy increased it could actually increase crop prices depending upon the exact details of the transition with the usual caveats about this only being a very simple calculation and all.

Also after having done some more reading I've learned that the real losses in efficiently are due to oxygen. Higher amounts of the waste gas in the system mean that sometimes instead of doing the right reaction it does one termed photorespiration that produces junk. So if you found a way to more efficiently exclude and remove O2 from plant cells it would greatly increase yields. So the problem isn't that plants reflect green light or whatever, though that could bump up yields a bit.