Things I'm keeping in mind:
BnN, and Amazon, are not the most accurate when it comes to pre-announcement info. They could have been placeholders, the price could have been inaccurate, even the titles! Who knows? Without a page count or description, I'm hesitant to determine the accuracy.
A $50 PHB is only for those that already know they want it as a resource - not for those "trying out the game." Not for the newbies who've never heard of D&D. Not for the long-time D&D veterans who are on the fence about 5e. This is what the Starter/Basic Game is for! And priced at $20, I have no problem imagining that this would be the loss leader. If it's a good product (if!), then (I imagine) a large percentage of people who enjoy the game will move onto the PHB with more complex options.
$50 makes sense to me for a really good core rulebook. Whether it will be really good remains to be seen. But high quality, good rules, dense text, and good art are all factors.
I could see the PHB priced at $50, with the DMG and MM at $40 each. $130 sounds better than $150.
Hell, I could even see a $50 PHB and a $50 DMG only, with the DMG having a monster section, with folios of monsters/NPCs released in the future. Why not?
The speculation is, of course, interesting. I would, of course, much rather pay $40 for a great PHB than $50. But I don't see anything to be up in arms about.
I just want it to be good.