Think of it this way, is a 4 carat diamond only double the price of a 2
carat diamond? The bigger it is, the more it costs and the increase in
price is a curve, not linear.

But that's assuming larger sizes are rarer. In the case of lumber,
shouldn't thicker boards require FEWER cuts, and therefore be cheaper
per bf? To a certain extent, of course - I understand that super wide
stuff is rare. I also understand that lumber prices vary a great deal
by location, but I don't know how that would affect 8/4 vs. 4/4
pricing.
Really I don't know much about lumber pricing and wood product
economics, just tossing out my ideas.

For something a little more concrete than ideas, I'll toss out the
link to my favorite lumber dealer, where bf prices for 8/4 are
generally $1 more than the 4/4 prices.
http://www.lakeshorehardwoods.com/LumberPricing.html
Oak, Peruvian walnut, and mahogany are the only species with a bigger
price disparity between 8/4 and 4/4, so maybe it's harder to find (or
to sell) 8/4 oak for some reason.

Andy