When you open a loot crate and receive a cosmetic you already own, it is automatically turned into coins. How much do you get, exactly? Is it a constant amount, a fraction of its unlock cost, or something else?
As Overwatch is still in the early productions, but a popular (and seemingly true) theory is that every duplicate gives roughly a fifth of it's price. For example, a 5C for a 25C spray or icon, 50C for any 250C epic, 200C for a 1000C Legendary skin, and so on. This is true for all things obtained with a preset price set on them.
This is supported, as in another Blizzard™ game Hearthstone there's a similar system. The cost of a legendary card is 1600, but if you get duplicate, you can disenchant it only for 400. The difference is that in Hearthstone, it's a one fourth, while in Overwatch, it appears to be one fifth.
There's an ongoing conversation about the topic over on Reddit that offers some more insight, but to answer your question, much proof as there is behind the theory, there's no "for-sure" answer that can be provided at the moment.