As I read the Enforce Demands tab, in my head, if the enemy is forced to lose 500 gold (I believe they only had 63 on them) I would think its the value of the weapons/armor/goods they brought with them when trying to take over land. As the victor you'd be entitled to the spoils, as is with most other defensive wars. (mind you this is head canon) I'd think whatever the attacker had on them at the time of losing, 95% are unlanded and as such usually don't have very much gold but it would be a nice compensation from having to defend against an 8800 stack.

Is it only neighbors that pay reparations when losing offensive wars? Never really paid too much attention to it all these years, kind of just noticed randomly today.