You have to realize the computer is going to demand whatever it thinks it can get from you. Here's some trading tips I have figured out over the last while:
1. If they won't trade world maps at first, trade territory maps with everyone on the board, then come back and deal.
2. Realize that they want resources as bad as you do. Don't be afraid to cut them off. Let them simmer without luxuries for a couple of decades, and then see how it goes.
3. Go to war and take a resource or two from them. Then deal.
4. If a Civ you're at war with is just about to be eliminated by another civ, demand anything you wish in exchange for peace. Chances are, they'll give it to you.
5. ....
I had a bunch more suggestions worked out last time I was playing, but I seem to have forgotten...

Oh well, those four are worth mentioning. Maybe I'll think of some more later.
My point was that the trading system can be manipulated if you just put some time into figuring it out. Many a time I have screwed the computer quite satisfyingly!