It's hard to determine whats reasonable and what not, it depends on both the pirate and the trader, the circumstances and both parties RP and of course the traders conduct.
I also don't think that the traders shiptype has to say anything about him being a new player or not, I've seen freighters with a CAP8 and Battleship License.
As Bacon pointed out, there are a couple of RP scenarios where being the trader just sucks and the pirate, no matter how rude and demanding he is, isn't doing anything wrong.
If you are on a Repex transport and you meet a Unioner, you are likely to loose either your cargo or your life. Same with LWB and Synth Foods, Lane Hackers and Ageira, Bretonian Privateer and Samura,...and so on.
These scenarios however, are worth a bit more RP than the generic 'U stop! Dropallcargo or dai!...kthxbai!' but if it is RPed out and the pirates motives are clear, everything is alright.
The casual trader...well, I try to make reasonable demands but don't expect me to browse a commodity list and get my calculator out just to be able to estimate what would be 'reasonable' to charge and the amount completely depends on the shiptype, ranging between 100k and 2 million credits (for non-miners).
Also remember, if you try to run, powering up cruise engines, thrusting away, maybe even make me loose another trader because of that, the price is skyrocketing, same with oorp babble, the use of 'u' instead of 'you' and other stuff like this, and threats about taken screenshots etc...
So yes, without the exact context of the situation in the topic starters post, its impossible to know if it was overcharging or not and it seems just like another 'OMG! I haz kill pirate Gunship in mah tansport!!' gloating.