And I'm still over here asking for 3 mil and they're still running from that. I have rarely ever asked for more than 5 mil a 5k'er that's full with the most valuable ore, but its true that if population is low at the time or drops for a while, encounters are harder to get, and the value of what you do on both sides goes up. If you think you will run into a trade ship often, I'd stick to 5 mil and try not to drive them too nuts. You'll be more likely to be paid each time, if that's what you're really after.
I suppose it depends on the exact situation. Independent pirates are usually just out for themselves. Sometimes pirating is from factions that have inrp motives and may make high demands because of political grievances or to draw a lot more attention from the lawfuls than usual. 20 mil even for a full 5 k'er seems a little steep even to me, but I'd perhaps consider it reasonable for groups like privateers (before, because of war) and maybe the Maquis (but last time I checked they can just engage transports, so imagine losing your 5k'er without them making any demands). If they demanded 20 mil in that situation you might be lucky to get off paying big instead of being used for a larger goal.
You also need to consider that snubbing a pirate can often make them less agreeable to negotiating later on. Smart pirates wouldn't try to get 20 mil each time out of you several times a day, odds are you'll just avoid bothering. So if you get a big demand you can always try to heckle them like it was stated or ask 'if I pay big this time without a fight will you be leaving me alone for a while or is this going to be 5 times a day?" If you plan to make 5 runs a day, a pirate might be ok to get 20 mil on your first run and leave you alone for the rest of the day. If you fight or run, they may be happy harassing you the rest of the day just for the heck of it!
I usually don't like placing limits on that but we could limit indi pirates/junkers to 10 mil per pirate per encounter, and allow factions that can pirate to demand full value or up to a limit (half?). And of course, spec op ID's that can pirate could be equal to the factions so SRP characters could be a way for an indie pirate to earn more ability.