Someone mentioned in a forum thread that ACKS (a system I know little about) equates GP to XP and vice versa, so the default value of a barrel of troll's blood is equal to the XP you would earn from killing enough trolls to get that much blood.
As I've mentioned, I interpret XP as reputation and confidence. You get XP from treasure because the townsfolk see you flashing around lots of gold and assume you must really be something, and that makes you feel more confident. Similarly, killing monsters gets you XP because word gets around, and because defeating a monster makes you feel more confident.
Both things together makes me speculate about doubting townsfolk and fake reputations. You could restrict full XP to bringing back pelts or severed heads of slain enemies. Trophies that don't necessarily indicate slaughter, like a necklace of goblin ears, yields full XP as long as your Charisma is greater than the Intelligence of the person you brag to, or if it's 10 or more for generic bragging. Otherwise, players get full XP on a 5+ on 1d6, modified by Charisma bonus, with a further +1 for some kind of physical evidence.
If players spread false rumors of slaying monsters, they can get half XP on a 5+ roll, but only if they brag to someone they're smarter than. Failing the roll means no XP, and possibly a reaction penalty in the future, or a sound beating.