Best I can tell, you have to give them a solid smackdown before they want to talk peace at any reasonable price. I've seen the Ai ask for large sums as you describe, but I've also been able to demand large sums from them. However, the latter only occurs when I've taken 2/3 of their planets and destroyed 75% of their ships kind of thing.
The situation you describe is hardly what I'd describe as a crushing defeat scenario, so I'm not surprised the demands are still high. From what I can tell, massive loss of power relative to you is what triggers a shift in the price of peace.
What game variables factor into it, I don't know. Diplomatic relations, other wars, personality and behaviour traits could all play a part. But it seems totally dependent on massive power shift right now.
