How much experience does a guild get per person for a boss kill?

The amount of guild experience awarded varies. It is depended on specific factors... (Source)

Guild xp for boss kills is scaled on a few factors.

• Boss level
• Normal / Heroic
• Amount of guild members present (more xp for raids, since they are limited to once a week)
  • 20/25 - 25 player raid
  • 8/10 - 10 player raid,
  • 3/5 - dungeons
  • 8/25 - Old 25 and 40 man raids (Vanilla, TBC, and WOTLK)

Straight from Blizzard's Mouth (Source)

The amount of experience gained scales, with 3/5 guild members granting 50% normal guild xp, 4/5 giving 100%, and 5/5 giving 125% guild xp. The amount gained from each boss scales with your level relative to the boss's level, with Heroic bosses worth even more (1.5x). It pays off to have a full group of guild members!

All of these assume you were in a guild group. The comp of the guild group doesn't matter (4 or 5 guildies)

• 5 man regular dungeon (24800 xp per person, with guild lvl2)
• 5 man heroic dungeon (37800 xp per person, with guild lvl6)
• 10 man regular raid (27800 xp per person, with guild lvl6)
• 10 man heroic raid
• 25 man regular raid
• 25 man heroic raid

I realize this is incomplete, but it's the best answer so far. It's from my own in-game experience. Feel free to comment with your findings and I'll add them here.