Cobra effect
Two thoughts:1. They should've reduced the reward over time. If the effort had been successful, there would've been fewer snakes, and less need to get the snake population under control. This makes economic sense event without considering this effect. If there's a million snakes, you really need someone to do something about it. If there's only 10k, who cares?2. This is why it's important to focus on the actual goal. The goal was not to kill snakes. The goal was to have fewer snakes. Killing snakes was just a means to an end. Actually, the goal wasn't even fewer snakes, it was to reduce the risk of dying from a snake bite. A successful anti-venom would've been just as much a solution as the eradication of the snakes. Of course it's a lot harder to measure, "Are you doing something which is likely to reduce the risk of me dying from a snake bite?" than "Did you kill a snake?"