This is the fight you should pick.

Diffrent from most of others who posted here.  I think you did the right thing.  This is the place to draw the line.  when there may be confusion, misunderstanding, you should find ways to talk to people face to face to find a resolution.  However, when this is blantant taking advantage of you, you have to put your foot down.  And what you said was nothing wrong.  If they pay you this much, you would do the work accordingly.  Very factual.