Its not a simple linear relationship, you have to consider the cost to make a 70 vs 60 and the shear volume of 70" sets shipped vs 60" sets. Gen-10 glass handling plants are optimized for the 60-65" sets, so going to 70" makes them considerably more expensive to manufacture.
That is true but we, the consumer, doesn't care about the cost. And with our pockets, we can dictate the price. The reason 70" + is still too high is because there are no competitions out there for Sharp. Samsung and Sony big screen TV prices are way too high. But with Vizio starting to come out with the 70" TV, I could see Sharp has to lower their price in the near future.
Pretty soon I'll be able to retire and just live off the rebate check that I receive everyday