Because unlike the gold reward for basically every other quest, they don't work off a standard leveled list.

I've looked at the quest screen and I can't make hide nor hair of it.

Halp?

A script handles it. Each time you complete a quest, a check is made against the character's current level, and a set amount of gold is awarded based on a level-scale, e.g. 1-10 may get 100 gold, 11-20 may get 250 gold, 21-30 may get 500 gold (numbers random as I'm at work so cannot check the script for the precise values).

...ok...I'm about to edit a script. OMG

Lets see if this works...

Edit: found it and it works! Thanks Vorians.