Sennheiser has gotten "more expensive" because they've gone to an enforced MAP (the prices you see today for HD 600s and HD 650s is what SRP has been for years and years and years). Like Grado. Beyerdynamic prices normally dance all over the place. Like Denon. Based on the market.
As far as the overall trend towards more models at the $400+ mark, that's a bit more recent trend. And you can thank market researchers and more directly, Beats, for that one. Basically market research from a few years ago started showing that headphones as a market/segment were not only profitable, they were showing growth even during the recession. Even the super expensive models ($350+). So of course more people want into that. There's profits. (And I don't count Bose in here, because Bose has had a $150 and $300 offering since the proverbial beginning of time and it didn't seem to do anything either way years ago; usually they were just regarded as a bad value and written off relative to cheaper offerings).
It also gets screwy when you have Beats products (which have dominated market share) and you want to position your product as an "audiophile" offering, and not compete with them. That sees an upward trend too (pricing is part of marketing). Beats is also unique because they've changed headphones from being an A/V accessory into a fashion item - like shoes or jewelry. End result is you get models like the Beyerdynamic T70, a "mid-range" headphone with an SRP of $669. Even five years ago it would've been laughed off the market. But today, they let it get "discounted" to $399 or $499 and it sells. That's what happens when "bad" headphones are $299.
Inflation is only a small piece of the puzzle though - we aren't seeing huge inflation. And the prices of other durable goods haven't skyrocketed as a result. This is simply market exploitation - it's a new segment that happens to be returning huge profits, and consumers don't really seem to have an issue with a $400 headphone even if it performs as well as something that would've cost $30 or $50 or $80 as little as ten years ago. For example, does anyone remember the Sennheiser HD 485?
Of course, this isn't universal. Some manufacturers have absolutely resisted this upward trend. Examples include Koss, Bose, Grado, and Ultrasone. Sony changes models too often to really make a case for, but they haven't danced too far out of the $50-$250 range in a while (they always have "one" over that, but not their entire product line - this week it's the Z1000).