If you have the toolbar, your local Tips guy will reveal on the drugs page the rough baseline of what the prices are in your city on average. Each district's prices are a few % higher or lower than the baseline price. Sooo when he says something like this:

Rumor has it that Georgie Mitchell has set Coke at $9,448, Heroin at $6,485 and Barbs at $2,982 for Philadelphia but your local dealers may charge you more or less.

That means if you see Coke in your current district at $9,200 then you know its a decent price and you probably will profit off of it. If you see Coke at $9,500 then you will probably not profit from it. You can use this to judge what other districts in that city could be at times when your family isn't there to share prices with you.