Yeah there is an unspecified limit on demands though, and it its a sanctionable offence to make "unreasonable demands". So unless someone asks you to drop all your cargo* you can never be asked to cough up your entire cargo's worth in credits**.
With those rules you will always profit when you pay the pirate. Some runs you will profit less due to getting pirated, but other runs you will meet no pirates and get your full cargo's worth, and in the long run the credit loss to pirate demands will be balanced out pretty evenly; compared to choosing to get smashed and reset to zero. Of course if you run into 2-3 different pirates on your trade route you can end up with no profit when you have paid them all, but you have to be either extremely unlucky or extremely incompetent for that to happen.
* Generally speaking I think pirates in transports are allowed to steal whatever they can hold, also when that means your entire cargo.
** Certain ID's have the dreaded "May treat as combat target" line on them and can demand anything they want and still blow you up. Those ID's are mostly lawful though, and thus mostly a problem when you are smuggling.
@SMG: Yes ok I was playing the devil's advocate. I do know the "15 million now, 3. 2. 1. Engaging!" type pirate, but at the same time I don't think piracy with high demands should be automatically written off as poor rp by the trader in question.
You wouldn't believe the amount of bad attitude you receive by traders when you pirate them, everything from "//this is rp server, you don't rp!" to PM whining to abusive name-calling.
I think it's important that we do our utmost to make players understand that piracy is an integral part of both the roleplay setting and the gameplay, and that pirate players aren't non-rp'ing douchebags by default, even if they ask for a lot of credits (within reason naturally).