Ya its a fine line, when it comes to contracts ive learned a few lessons the hard way as well. When i used to do illustrations i had something simialr pop up when they wanted the characters rough sheets to go with illustrations which was not mentioned or in the contract. So after that i started to have everything written down in the contract as far as concepts, roughs and final images..

I also use the same practise with 3d models from sketches to finals to who gets what and if i can use them in my demo reel.

Always make sure you get paid for your time, lot of people dont have a clue how much time is involed in producing what we do. Again when i was more of a illusrator i would charge for new supplies as well like markers, papers exta.

Its hard to mentions and you dont want to scare off potential work but you have to take care of your self as well.