I apologize if this is old news but I really haven't seen this subject covered lately and there have been some questions about the amount of tribute it takes to get a computer player to ally with a human player. As most forummers play multiplayer it's understandable why this topic doesn't come up. Anyway, I did some informal tests and found something that surprised me a little. As Sandy Peterson mentioned a while back in Heaven, the computer opponents who can be tributed into alliance are the ones who do not set diplomatic stance with you to "enemy" at the start of the game. The number of potential partners depends on the number of computer players in the game. Against one or two computer opponents, you will be unable to ally. Against three computer players, one will be willing to ally if given the proper amount of tribute. Against five or six, two will be willing to ally. Against seven, three will be willing to ally. Now for the part I found surprising: the amount of tribute necessary changes with the difficulty level set (not really surprising, but not exactly AoE 101 either.) Here are the results of my tests. I should point out that these were all "lump tributes." In other words, I saved the game before tributing and kept re-loading and tributing a different amount to find out the necessary sum.
Easiest: 368 resource points
Easy: 507 resource points
Moderate: 711 resource points
Hard: 1291 resource points
Hardest: 1769 resource points
These amounts were arrived at by using food to get to the nearest 100 points and stone to make up the difference. I didn't check whether other resources in other amounts would create an alliance but I don't think that's the case.
Also, even on hardest it is possible to get three computer players to ally with you but you're going to be paying a ton of tribute for the privelege.
I should also note that all tributing was done in Tool age at around the five-minute mark. If there is a cutoff point where alliances won't be accepted, I'm not 100% sure where that is.
See you on the zone.