A few things, First stop with this "a truck driver wouldn't carry that much money" because one thing alot of people here seem to forget is that we are playing a game, and it is not always possible to make the game fit with the real world. If it did that trader Ze'ev killed running diamonds into Kyushu wouldn't show up again with another load 8 mins later. He would be dead and would have to start a new character.
If I had my druthers, the Farmers would have a terrorist ID and would destroy all foreign traders in Kusari. Thats not going to happen though, and therefore we have to adjust our RP to fit the realities of the game and rules and also try and keep it fun. The real way the Farmers should RP is to demand all trader drop all there cargo and have it destroyed in front of their faces, 25k from their destination....but then we would have a thread about what portion of cargo is allowable in a demand.
Not all traders are equal, and therefore not all demands are equal. It is the circumstances that make the most impact on what I demand. A synthfoods ship (If any ever came to Kusari:() is going to get a much larger demand than say a Kishiro or GMG ship, or a generic trader for that matter. A junker transport, or IC, or Outcast will also get a bigger demand, same with a Bretonian shipper. I do take the players level into account, but I generally don't pirate traders under level 55 or so anyway, so it's not really a problem for me. I have asked for a few rather large amounts in the past, but they are all in RP, and when I do it, I remind the trader why, I want it to become so unprofitable to run goods to Kusari that they will stop coming. It takes more for some people than others. I think I even demanded 10 million from Tenacities Junker once:P. To paraphrase Xoria in a sanction thread recently, the right amount is the amount that hurts. If it doesn't hurt your pocket book, then what have I accomplished, because I am not in it for the money.
In conclusion, don't tell me how much to ask for when pirating.