When I pirate I usually somewhat try to judge a player's experience, and their level is usually a good guide. Depending on the value of the cargo I usually don't demand more than 5 million per instance. Less if they seem newb or genuinely broke. However player faction tagged ships I always treated a little differently. Regardless of it all, if a lvl 80-90 wants to refuse a fairly reasonable demand, I don't feel any guilt in opening fire. Then of course there are other role-play considerations I took into account when pirating certain players or factions, which would increase the price of a demand.
I know even 5 million seems low, and in a way, for one encounter it is. Yet we still run into times when you have more than one pirate hitting a ship, either at the same time, or one encounter follows another shortly after. Traders can wind up being pinched multiple times, so with average players of average level and equipment, I keep it to 2-4 million per demand on average. Reasonable demands are more likely to wind up being paid. Someone out to pirate is usually out to get paid before having to fight. I'd rather pirate 4 ships for 3-4 mil each than pirate one of them for 20 and wind up fighting and (likely) losing the ship to a base. If traders know you keep your demands low they won't be so afraid to cross your path again.