Campaign scoring

Campaign Scoring, also known as Meta-game, is an optional game feature for the campaign mode of Halo 3.

When playing with Campaign Scoring turned on, the player plays a normal campaign mission (single player or cooperative), but progress is scored with points for notable activities. Having skulls activated increases the amount of points gained, as well as accuracy and kill count. Different enemies give different points as do headshots (and assumably any other kill that would be worth a medal). Betrayals will cause negative points so cooperative work will be necessary. To find out more about campaign scoring, listen to mission Design Lead Paul Bertone on the Bungie podcast. 

Point Values
In the Meta-game you earn points by killing enemies, here is a list of their base value (what you get per kill without any extra modifiers).

Enemies
Flood

Specialists
 * Pure Form Stalker - 20
 * Pure Form Ranged - 30
 * Pure Form Tank - 50

Infantry
 * Flood Carrier Form - 10
 * Combat Form (Human,Brute,Elite) - 10

Covenant

Heroes
 * Brute Chieftain (Weapon/Armor Variants) - 150

Leaders
 * Brute Captain - 50
 * Brute Captain Major - 50
 * Brute Captain Ultra - 50

Specialists
 * Brute Stalker - 30
 * Brute Jump-pack - 30
 * Brute Bodyguard - 30
 * Jackal Sniper - 20
 * Grunt Heavy - 10
 * Grunt Spec-Ops - 10
 * Hunter - 150

Infantry
 * Brute Minor - 20
 * Brute Major - 20
 * Brute Ultra - 20
 * Grunt Minor - 10
 * Grunt Major - 10
 * Jackal Minor - 15
 * Drone Minor - 10
 * Drone Major -10

Forerunner

Infantry
 * Sentinel Aggressor Minor - 5

Vehicles
Light
 * Shade - 50
 * Ghost - 100
 * Chopper - 100

Standard
 * Prowler - 150
 * Banshee - 100

Heavy
 * Wraith (Both Variants) - 200
 * Phantom - 250

Giants
 * Scarab - 500

Multipliers
In the meta game, the difficulty selection, active skulls etc. This will help you determine the score multiplier for that session.

Difficulty Multipliers
 * Easy - 0.25x
 * Normal - 1x
 * Heroic - 2x
 * Legendary - 4x

Skull Multipliers
 * Iron - 3x
 * Black Eye - 1.5x
 * Tough Luck - 1.5x
 * Catch - 1.5x
 * Fog - 1.5x
 * Famine - 2x
 * Thunderstorm - 1.5x
 * Tilt - 2x
 * Mythic - 2x

Time Multipliers 3x to 2.5x to 2x to 1.5x to 1x

It should also be noted that in order to get the achievements, it should not be done in co-op as each player will have to score the quota of points.

The Formula
The Formula for Campaign Scoring is:

Points Earned = Unit's base point value X (Style Multiplier + Occasional 2nd Style Multiplier) X (Difficulty Multiplier + Skull Multipliers)