I'm going by what my husband had to say on the subject; he's pretty savvy to this. He talked to the insurance company and looked up the drugs and judges that the cheaper drug is certainly worth at least discussing.
Sometimes you have drugs in the same class that work in different ways. Thus, Paxil, Prozac, Welbrutin etc. are all classified as anti-depressants. However, anyone who has been treated for depression can tell you that they are not interchangeable.
Sometimes you have an old drug that is going off-patent. The drug companies get around this by tweaking the formula or even just the delivery system and market it as ****NEW****. Sometimes it's worth the extra money to pay for an extended release dose. Sometimes you're just paying big money for a single pill that combines two inexpensive drugs.
I am dealing with several long-term chronic conditions myself. For some scripts, I have to have the name brand. For others the generic does just fine.
I guess the takeaway is that the patient has to be proactive and engaged.