In most strategy games, if a score victory is possible, when it comes to alliances the score is usually an average of the scores of its composite members, or some other similar formula.  This prevents a situation such as the following (I'm using round numbers to roughly demonstrate the end of my last game)

8 player game, turn 299 

Aliance A

Player 1: 2500 

Player 2: 1500

Player 3: 1500

Player 4: 2000

Cravers

Player 5: 1000

PC

Player 6: 6000 

Alliance B

Player 7: 500

Player 8: 500

In the above example, player 6 is clearly whomping the others.  They've been running solo, because alliances with NPCs right now are borked; they'll randomly declare war on neighbors that are way stronger, or will declare peace right in the middle of a war that you're winning, preventing you from taking that planet with the quadrinix you need. 

 Players 1-4 are in an alliance, players 7,8 are in an alliance, and player 5 is cravers, so can't form alliances.

At game end, the alliance wins a score victory with 7500.  Does this seem right, when their average score is actually only 1875?  Other victory conditions such as wonder, conquest, and economic have their required win number increase in alliances, due to the obvious advantage.  Why should score be any different?

(Yes, I know that I could have easily won with a different victory condition, but I'm trying to go for the score victory achievement in this case.)

I know.  I'm not arguing for a pure average, because you're right, that detracts from the value of an alliance.  I'd argue that the formula should be something like the score of the alliance leader (That is, highest score in the alliance), plus the average of the other members, or maybe some sort of diminishing return after the first.  This would be similar to how in a game that normally requires 4 endless techs to win, in an alliance with 2 members the number goes to 6, not 8.  

With the above suggested forumula, while it does mean that for a 2 member alliance, you'd still just have a sum, but then you wouldn't have score bloat for 3 or more.  Thus it's possible that a single player doing extremely well could still beat the alliance, if they're just flattening them utterly.