Saving $15 on 8 million units = $120m.
It doesn't really even matter, if the cost difference is anywhere near $15. Manufacturers try to shave as much cost from the device as they can get away with. And there are multiple variables that Sony needs to balance. Most important of these are volume and margin.
Let's assume that Sony does not want to sell consoles at a loss, which locks the minimum retail price at manufacturing cost. Retail price affects volumes: there is a significant difference between $399 and $419, for example. (And $409 is simply a dumb price, nobody goes with that.)
Suppose that Sony could have squeezed the price to $399 with UHD BD, but with zero margin. It wouldn't matter a bit, how many more sales they got, because N times zero is still zero. If they sell 5 million UHD'less Pros with $5 margin, that's $25M. Much more than zero, isn't it?
Now, if Sony were able to make profit from Pros with UHD BD included, some more estimates were required. Like, what is the increase in volume. Supposing they could go from 5M to 6M (which I highly, highly doubt), and the difference in margin is $2 to $1 (generous assumption, right?), Pro without a UHD drive would still make $10 million to $6 million of the "better" model.
Of course, Sony could adjust the retail price, but higher price usually leads to lower volumes, which affects total profit.
I genuinely believe that Sony has a bunch of pretty smart people who are better informed about consumers' price sensitivity, BOMs and effect of features on volumes than I am. Heck, I'd be willing to bet that they are more informed than most people discussing in this thread.