Time Orientation of Costs
Relevant costs are costs that will make a difference in decision-making and will need to be considered. In other words, it's a cost that hasn't occurred and needs to be considered!
For example, pretend you are a property developer and you can do two options to do an existing property. You could demolish the existing property and build a new house or renovate the existing property. The relevant costs would be the cost to demolish the property and build a new house or the cost to renovate the property.
Sunk cost is costs that have already occurred and are not relevant in future decision-making. In other words, it's a cost that has occurred in the past and is not relevant since you can't change it! For example, consider the above example with the construction of a new house or renovation to an existing property.
The sunk cost would the cost of construction of the existing property as the cost has already occurred and is no longer relevant in decision making.