They are overpriced, yes. But the rates very A LOT depending on the season.
I'm looking at going March 10-13, and the Disneyland Hotel is $176 a night, versus $100 a night at the Hojo or Tropicana. For $76 difference a night its worth it to me.
Now, if you book even a week later, the Disneyland Hotel is suddenly $270 a night. The Hojo and Tropicana are still $100 a night. $170 a night difference; not worth it to me.
Its all down to what you feel comfortable paying anyway. I don't think its overpriced. The security (real or perceived) of not having to leave Disney grounds and having my car parked in a Disney parking lot gives me peace of mind. To others there's no difference. Neither opinion is more valid than the other.
I wouldn't even include the Grand Californian with the other two in this discussion. Its a much nicer, much newer hotel than the other two.
And if you want overpriced, look how much the Grand Floridian is for a night March 10th. Garden View room (I believe the cheapest room) is $529. Wowzers!