I'd say three million is around the top I'll demand from a normal trader (ie standard goods). I tend to stick around the 1 to 2 million mark though, or less if the trader is empty, or if my mark is a fighter or transport pilot.
If you're hauling ore, you'd better have an escort with you, 'cos I'll be taking you for quite a lot, although I'm usually pretty fair. I'll either take a chunk of money and a smallish amount of ore, or I'll ask for a percentage of your cargo. Either way, it'll probably be a fair way above the 3 million mark, cos lets face it, that ore sells for 11k to 16k per unit.