But that's a bit deceptive, because you could have paid $90 for all three in 2014, but you (presumably)bought from an FLGS.
If I remember correctly, the MSRPs of the PHB by edition were:
1E (1978): I think it started at $9.95 in the 70s but was $12.95 (?) by the time the orange spine version came out in 1983. I know this went up to $15 by 1987.
2E (1989): $20.00
3E (2000): $19.95
3.5E (2003): $29.95
4E (2008): $34.95
5E (2014): $49.95
Interesting to note that the MSRP of the 2E and 3E PHBs were the same, 11 years apart. Presumably printing costs went down? Or maybe the rise of the internet?
3.5 is partially inflated because it became a bigger book, or maybe 3.x was too cheaply priced.
That 4E to 5E jump is the most worrisome - that's a 42% mark-up in six years, or 7% per year. Isn't inflation on average about 2% per year? If the same pattern holds and 6E comes out in, say, 2024 for the 50th anniversary, that would would be 70% mark-up (7% per year) and an $85 MSRP.
On the other hand, if they go the evergreen route they'll have to do price increases more incrementally. And I suppose that also justifies a new print version, with new covers: maybe in a few years in 2020 or 2021, with a new list price of $59.95. Just a guess.