From my experience so far (with the few derms I have seen regarding hairloss) some of them really don't know much about hairloss. They probably just know enough to prescribe propecia and tell you about rogaine. I'm definitely not trying to talk negatively about dermatologists. It's just that they have SO much that they have to deal with that they can't be expected to be experts on everything. That being the case I feel we need to take the reigns when it comes to dealing with hairless and our doctors. explain WHY it is important for you to get proscar instead, tell them that you plan to quarter it. Then tell them, when it comes down to it, that this will save you ALOT of money.
When I talked to my doctor about. I had to explain all this. She had to go confirm with a colleague to see if it was ok then came back in the room with a prescription for proscar. which I then took to Walmart and paid 9 dollars for four months of finasteride.
If your Doctor is not understanding about this, especially when they know how much money it will save you, then maybe you should find another one. And don't be afraid to let them know why you are looking fr a new derm.