I'm a bit confused. These terms come from two quite different disciplines.
Contribution Margin is used in management accounting and is the marginal profit per unit of sale. I've never encountered contribution margin in my (admittedly short) career in project management.
Earned Value is a concept that is limited to earned value management, which is very definitely employed in project management. Earned value management is a mechanism for measuring whether work (effort) is proportional to schedule(estimate).
Moreover, I can't conceive of a realistic situation where earned value is negative. (I can imagine contrived situations, but none that are plausible). Furthermore Earned Value most definitely does not represent cost.
I suspect that you need to revisit the concept of earned value management.
There is a relatively decent example worked in at pmis. The following pages provide some additional resources & explanations.