So this comes down to actually being a good manager. It's hard, and lots of people do it wrong / pretend they are good but aren't / etc. Ask yourself what you really want in a developer and then manage your team to that standard understanding that each member has their own strengths and weaknesses. Something like:
- Elegant and easily understood code
- Good at estimating and meeting deadlines
- Productive and participative in scrums
- Thoughtful and supportive of alternative views
Coders are people. They are a unique breed of people, sure, but if you want to gauge their worth, then you manage and treat them like people. Not monkeys at a typewriter. A small group of talented and creative coders can save a company millions in just a day of work. I've seen it. You need to appreciate their value by paying attention, not coming up with some arbitrary metric that makes your job easier.