Lynnfield is the 45nm Core 2 Duo, kind of old to make a comparison. Intel doesn't sell anything as big as 300mm^2 in their current line except for $2k monsters like Westmere EX.
I honestly don't know where I got that number from. I was thinking something like $200 average for the 2-core Sandy Bridge die, which is 90mm^2 I think. I'm obviously not privy to sales volume numbers nor to what their wholesale prices look like. So feel free to apply a factor of N to that intuition.
And obviously I meant revenue, not margin. Apologies.
But then you have things like the Core i7-980, which is also about 300 mm^2 but retails for up to $1200. I don't know what the average is, but $1-$2/mm^2 is in the right ballpark at least.
A mid-level Lynnfield CPU of about 300 mm^2 would mean a margin of $600, but the processors sell for typically $200-$500.