"The general principle is that two things that enhance Intelligence won't stack with each other, but something that enhances Intelligence and something that enhances an intangible nearby thing, like the wine does, or enhances - luck, ease, like Ismat's loupe -- do. Of course, Cheliax has access to the wine too, this isn't a relative advantage, though I think it'd be much more inconvenient for them to send people to Osirion to buy it and if they have their own casters make it it'll be weaker. The spell isn't hard to cast but the effects are tiny if it's not someone as powerful as Nefreti casting it. 

The obvious way to compare to a headband would be to ask people how much more they'd pay for a headband, but I don't know anyone to have tried that."

"It's one gold for a drink, if I recall. The general principle is that two things that enhance Intelligence won't stack with each other, but something that enhances Intelligence and something that enhances an intangible nearby thing, like the wine does, or enhances - luck, ease, like Ismat's loupe -- do. Of course, Cheliax has access to the wine too, this isn't a relative advantage, though I think it'd be much more inconvenient for them to send people to Osirion to buy it and if they have their own casters make it it'll be weaker. The spell isn't hard to cast but the effects are tiny if it's not someone as powerful as Nefreti casting it. 

The obvious way to compare to a headband would be to ask people drinking the wine how much more they'd pay if it worked like a headband, but I don't know anyone to have tried that."