I made some weird peace deals in a recent game. Firstly, I was trying to culture-bomb the Torians. I built a culture palace base in his system. He demanded I give the starbase to him. I refused, and he declared war. I then offered him peace if he'd give me some money. He agreed at once - my miltary was a lot bigger. I repeated this several times.
Later in the game, my allies attacked a minor race - the Carinoids, I think - and I was forced to join in. Again, I tried to offer peace immediately. The Carinoids were willing to pay me 2,000 a turn for 50 turns in exchange for peace! 100,000 credits for not taking part in a war I didn't want to fight anyway has got to be the best deal I ever made...
So I started out as a pacifist and ended up as an extortionist. And the Carinoids ended up with the biggest debt in galactic history.
This is a well-known problem. I don't know if this was fixed in the expansion. Enough people have made this observation that it must have been.
This also works if the AI players have an alliance against you. Suppose the Yor and Drengin are allied, and you get into a war with the Drengin. The Yor-Drengin alliance forces the Yor to declare war. You start winning the war, and now negotiate peace with the Drengin in exchange for concessions. You remain at war with the Yor, and in a few turns the the Drengin declare war again, because of the alliance. Now, you can negotiate peace with the Yor in exchange for concessions. You can rapidly bankrupt the AI players this way.
This only works if your already have a superior military, and were headed for victory anyway.