I do think this is fraudulent, and I think the comparison falls down: A customer buying the 20$ dropbox account knows exactly what he is spending his extra 10$ to get, and it is worth it to him to do so. Even if objectively, 10$ may seem like excessive premium.
Sennheiser in this case is being fraudulent by claiming that there is an actual difference between the two models while in reality they are exactly identical apart from a tiny piece of foam clearly put there only to defraud customers that want better sound and are prepared to pay for it.
To quote the amazon page for the 595's:
"The HD 595 is the top-of-the-line headphone of the audiophile 500 series range, boasting a new level of comfort and sound quality. Features include our E.A.R. technology and a highly constant,compressed cellulose fleece to reduce total harmonic distortion. Velour ear pads and high-quality leatherette headband both provide outstanding comfort."
compared to the 555's:
"A comfortable, high-quality headphone system"
Don't you see how this is fraud at work? Nowhere in the description of the difference between these models is the piece of foam mentioned or hinted at. To the contrary, the description of the 595's indicates that they include ADDED technology to improve sound quality and comfort. Which is full-on lie.
No, pricing is not connected to incurred cost. That's not the issue.