Is there any good reason why they should sell it for $40?
Well... there's an invisible curve that relates price points (and profit per unit) to the number of purchases - in broad terms, as the price goes up so does the margin but the number of customers goes down (but it's not a 1-to-1 correlation, and sometimes a price increase may even lead to
more sales, bizarre as that is). I say it's an "invisible curve" because it only has two known points - if the unit is priced at $0 you get no profit, and if it's priced at $VeryHigh you get no sales and so no profit. Everything between those two points is (largely) a matter of guesswork. (There's a comparison I could give, but it's political, so I won't.

)
So, WotC should aim to find the 'sweet spot' where they maximise their profits, allowing for those trade-offs - the point where "margin per unit" x "number of sales" is maximised. That may mean a PHB at $40, or $50, or some other value.
But, speaking on a more personal level...
I was very surprised by the $50 price (again, assuming it's accurate). I had assumed 5e would look much like 2nd Ed ("black cover" version"), 3e, 3.5e, or 4e - that is, three books of about 300 pages each. And, since the price of the books has been reasonably stable across those editions, I expected it to stay much the same again, at approx $40 for each of the three books. Given those assumptions, $50 is indeed a big
above-inflation increase.
Now, there are three things to note about that:
- We don't know
what else has changed. (I mean, technically we don't even know the $50 value is accurate...) So, it may well be that instead of three 300 page books, 5e takes the form of one 500 page book. Or it might be that it's three 500 page books, and that they're so complete that there's just no call for supplements for the game, ever (unlikely, I know!).
- Traditionally, the core rules for D&D have represented absurdly good value. My 3.5e core rulebooks are probably my single best purchases
ever, closely followed by my 2nd Ed ones - in each case I used them extensively for years, running multiple campaigns. They would have been very good value at twice the price. So it may well be that WotC, recognising the disproportionate value of the core rulebooks, and also recognising just how badly supplements fare by comparison, feel that those core rulebooks should be more profitable. Honestly, I can't fault their thinking in that regard, if it is the case.
- There is, of course, a saying about making assumptions. WotC never said 5e would follow the previously-established models and price points. Which is why I'm
surprised by this 'news', not
angry.