tell him what you just wrote (about it being so expensive) as well as reminding him that it's the exact same active ingredient. funny that you mention this as i just went to my derm on wednesday and asked him to prescribe me proscar. he did no problem!
i totally hated this guy the first time i went in (he pretty much blew me off) but this time he was nice and was knowledgeable about both propecia and proscar and just told me that i would need to get a pill cutter and that i should watch out for any side effects since the dosage would be going up. also, he mentioned keeping the cut tablets away from any women (especially if they're pregnant), since the protective coating on the pill will be useless once i split them apart. this is because finasteride can potentially cause deformation in the genitals of a developing fetus if ingested by a pregnant woman.
good luck! if your derm turns you down, be sure to put up a fight since there's absolutely no reason why a derm should deny such a request.