When a client goes to a developer, it is not the developer's responsibility to inform the client of the competitor's offerings. You provide the best solution that you offer, for the individual's needs. I used Drupal to add value to the site without adding cost because I felt it was a good fit. If they need a more custom solution, I would not bring Drupal into the mix at all.
If someone takes a $200 pre-built and sells it for $2000, that's one thing, but if that same developer figures that the time and effort needed to produce a custom solution would require the amount of time billed at $2000, that's another.
The problem comes when the client is not in a position to make a fair comparison between off-the-self and custom.
Unfortunately this illustrates the exact perception we are trying to avoid: