On One hand you say the scroll is expensive because it's so hard to get, while on the other hand you say it's as easy as 3-4 BCNM's. Not taking into account whether or not your group goes with the "your orb your drop" or the "let's spilt everything 3 ways" even though it's a fair chance these are pickedup players that you may never have seen before and you expect them or yourself to be trust to sell everything and give each other fair shares.
I, of course knew I was giving an extreme example with the milk thing. I was just saying these scrolls prices have such a huge range 100k~800k that something is fishy. Normal economics aren't at play, if it were players would have seen a slow and steady climb in price not 100k or more price jumps.
I have also done these BCNM runs and spell farming runs and haven't come across a single scroll sometimes days at a time. Or in the case of BCNM, the drop went to someone else.

I tend to look at the market values in a general way. Namely, level to spell or a progression where higher spells become harder to "get" and cost more if available in NPC shops. Most of these "rare" scroll are around the 37-40 mark and if anything drain Gil from other items like ninjistu items and or other choices.