And old people my pharmacist was telling me when I was pregnant (and getting some vitamin d supps) that he sees the dame scripts come in all the time, for a really expensive vit d, and all the old people refuse to get anything other than what their doctor wrote down, so they end up spending half their pension on brand name medications.
Vitamin D formulations vary widely, and the form that it's in determines how well it's absorbed and used. Could be the more expensive is actually a better product.

I make the decision on a case by case basis. There are real problems with some generics. Best example of this is the horrendous side effects of on many of the generic tamoxifens. Women have suffered awfully without realizing it was the interaction with the fillers and not the drug itself.