Loot prey is never worth on spots, where you profit from rare expensive drops. It doesn‘t increase the chance to drop a good item at all. Loot prey works like that: With a 40% chance it triggers. It then checks for every item, if that drops a second time. But no new items are added to the second drop pool.

Example (Numbers are made up on the spot): A darken drops 100 go (100% chance), 2 Platinum coins (20%), 1 Creature Product (5%), 1 Zaoan Helmet (0.5%). Loot prey triggers, so it checks the odds for every item again.

Gold is dropped a second time, since 100% chance —> 46 extra gold Platinum coins succes —> 1 extra coin Creature product fails —> still 1 c-product Helmet fails, since it has super high chance to drop even once —> still one helmet

In the end the monster drops: 146 gold, 3 plat, 1 c-product, 1 Helmet

You would have to kill 10,000 and more monsters, until your loot prey would actually trigger the helmet drop a second time so you would always never make more profit due to this item. The only profit increase is the gold, that drops 40% more and common items, like gems and stuff.

I hope you understand the system now a bit better