Difficulty multipliers:

Easy: 0.25x

Normal: 1x

Heroic: 2x

Legend: 4x

Multipliers(for the kill)

Grenade stick: 1.25x

Headshot: 1.25x

stealth kill: 1.25x

Road-kill: 1.25x

Multi-kill: 1.25x

Needler superdetonation: 1.25x

EMP: 1.25x

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 bonus:

0-15min (3x)

15-20min (2.5x)

20-25min (2x)

25-30min (1.5x)

they are on this site

http://www.microsoft.com/silverlight/halo3.aspx i typed them up for easy copy and paste action because the site is in silverlight.