Is there an imperative reason to work towards this balance? Why not let animal mobs be less desirable to fight? It could introduce new decisions into the game, allowing for players to avoid fights when they wish rather than feeling obliged to kill everything "for the loot". This gives you new balance levers to pull, as well. EG: Quests that venture into the sewers could now be harder because the rats down there down't drop any useful loot, meaning more preparation is needed before heading in.
That being said, you mentioned trade goods. If the players require food, having animals drop meat and such could make them attractive targets depending on how hungry the PC is. Assuming there is magic in your fantasy setting, perhaps magic-using players of a necromantic bent could "harvest" soul-energy from anything they defeat. Certain animals could be very valuable in soul energy even if they don't have loot.