The way my guild/alliance has done Zero Sum RKP (Raid Kill Points) is to assign values to every item. Take Nefarian for example. We assigned a 90 RKP value to any chest set item he drops.
The entire raid gains 90 RKP, split evenly among everyone. Since he always drops two chest pieces, that's 180 amongst everyone, not including any trinkets or rings he may have dropped.
So take your 40 people and divy 180 points amongst them. Everyone gets 4.5 points, even the people who will be 'buying' the item that dropped. (Our guild also awards RKP to people who showed up and were ready to go, but couldn't fit on the roster)
Now that the raid has made their RKP, everyone who wants it can pipe in that they'd be interested in the chest piece, usually through tells to whoever is handling loot (officers, etc) and whomever has the most amount of RKP will then pay the 90 RKP for said chest piece.
This way you have 180 points going in... and 180 points coming out. Remains balanced at 0.