$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.
It
really depends on what you're getting for your money. If the book resembles the 4e PHB, then anything over $40 is excessive (the 4e PHB was $35, which adjusts for inflation to $38ish). Heck, even the 4e PHB is pushing it, given the fairly large font size, the wide margins, and the frankly excessive use of whitespace.
If, however, the book resembles the Pathfinder Core Rulebook or the Numenera main book, then $50 is entirely reasonable - those are both pretty thick books with decent art, and suitably dense text. (And, given RPGs that have released in the last few years, this is more what I'm expecting anyway.)
But one of the real oddities of RPG pricing is that you can't actually judge the value of the offering by the format of the offering itself. A book priced at $100 might be absurdly good value, while one priced at $10 might be a total rip-off. And the quality of the artwork, the covers used, the text density... even the quality of the writing are all largely incidental to the value of the thing. The 3.5e Core Rulebooks would have been great value to me
at almost any price, because I played the hell out of that edition.
I could see the PHB priced at $50, with the DMG and MM at $40 each. $130 sounds better than $150.
My concern is less with the price of the thing, as such, and more with the sheer weight of rules. A $50 PHB suggests 500+ pages, and if you add to that a DMG and an MM as well... well, I feel it's just getting too big. (Having said that, the possibility I hadn't considered was that the "PHB" may, in fact, be
the game, with everything else being strictly optional. In which case, I would be very happy indeed.)
But, when it comes down to it, the question is less "what does this cost", and more "how much will I use it?" So...
I just want it to be good.
Amen.