Ask YC: What are you working on right now?

There have been quite a few studies that indicate that trivial reward systems tend to backfire, and have unintended side effects. These are definitely trivial rewards for good developers. Have you instead considered just implementing the measurement system, without the rewards? People can then see their success rate and have clearly defined goals for doing the work you want done. Then, perhaps take everyone out to a really good lunch, or have a catered lunch, at the end of a successful week of pruning? It's no more expensive than your reward system, and feels more like a prize to most folks--everybody says they like cold hard cash, but once basic needs are met, things like the work environment take on more importance. For example, Google pays lower salary than almost any other employer in the area, but it is the "best place to work" in the country.

When offering large enough rewards, you can convince people to do things they don't want to do. And when you ask people nicely for help, and give them clearly defined goals, you can also get them to help without a reward--they'll probably even enjoy the work more, because the human brain shoots out good chemicals when working within a community for a common cause. but, when you offer them a trivial reward, you both offend them and only grudgingly get results (with unintended consequences like code that gets written with the intention of claiming a reward later to clean it up), and morale will go down. Humans are fickle creatures, and I think you're setting yourself up for a long painful lesson in human resource management.

Just a humble suggestion.

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