When negotiating the sale of trade rights, map information and alliances, start with a high amount first (say 25,000 D). The AI will offer a single payment ranging from 800 to sometimes 4,000 D or more. Resubmit an offer that is 2,000 D lower than your starting offer... and continue doing so until the AI suggests regular tribute, like 2,550 D for two turns. This shows you how much the faction can pay per turn, in this case, 2,550 D. Make a final submission of 2,550 or the tribute amount but make it for ten turns... I haven't tried higher. Most of the time you will get the final submission, which in this case, was higher than the first amount requested -- 25,550 D.