For Robert, the coin is a symbol, and it's a symbol of the relationship he had with Beulah. It's a symbol because it represent things beyond monetary value, and only has value to Robert in light of what it signifies about his life with Beulah. It's cut in half, so it can't be spent. Even if it could be spent, ten-pesos has never been much.
On another level, as a coin, it's meant to represent money that is held in trust elsewhere. A coin is a form of credit, or, a symbol of "real" money. This is interesting because it shows how people often find and create symbols in their lives, and how symbols are used for practical purposes, in the real word, not just in literature. But as we were saying, the coin is valuable for Robert because it connects him symbolically with his wife. It gives him something to remember her by. It's a substitute for the real thing.