Correct: in 1977 the PHB could be had for $10, the MM was the same. The DMG was $15 when it came out in 1979. Correcting for inflation, that makes the DMG cost in 2014 dollars....$48.98.

The 2E AD&D book were $20 in 1987 which, when corrected for inflation is worth $41 in 2014.
3.0 PHB was $20, but iirc that was a loss leader. The later books were $30, I thought. That's $27 and $41, respectively.
4E, at $35 apiece, translates into: $38.54.
So while The books have certainly increased in price, that increase is not nearly as dramatic as it first appears, once you factor in inflation. And for that you're getting a full-color high-glossy book that is twice the size of the original AD&D books. In a world where paper costs have soared in the last decade. That's not the worst value in the world, IMHO.
To contextualize for those too young to appreciate the numbers, consider this: the average cost of a gallon of gas in 1979 (1st DMG) was 0.86. That's eight-six cents, yes. And that's following the OPEC crisis and a 20 cent rise from two years earlier when the PHB came out. In 1987, the average cost of a gallon of gas was 0.95. In 2000, that gallon was $1.17. (Of course, after that, things went a bit crazy).
Another comparison: average movie ticket prices:
1979 (DMG): $2.50
1987 (2E): $4.00
2000 (3E): $5.50
2003 (3.5E): $6.00
2008 (4E): $7.10
2014: (5E) $8.34