Identifying relevant cost

Relevant cost is important for decision making and identifying it actually requires some common sense. In general, relevant is always future, incremental cash flow.

Future
Therefore, sunk cost (past cost) should be ignored.

Incremental
The key question is "Is there extra cost making this choice?". When there is more cost to be incurred such as hiring new labour, it is a relevant cost. Fixed cost (like an unavoidable cost) is not relevant but if there is an increment, the increase is relevant.

Cash flow
Therefore, non-cash flow cost such as depreciation and provision should be ignored.

Furthermore, variable cost (avoidable cost) will be relevant for decision making. A committed cost, although is a future cash flow, is not relevant as it must be incurred, cannot be avoided even if the decision is not made. Opportunity cost (the sacrifice from choosing this decision instead of another) is relevant as well.

Relevant cost for material
This requires more common sense.
Example: Company has 100 units of material in inventory and need another 200 units for the job, current purchase price is $10 per unit. Such material will not be replaced. Such material can be used for another job to earn a contribution of $15 per unit and it has a net realisable value (NRV) of $18 per unit.
Solution: The problem is dealing with the 100 units, 200 units will be purchased so relevant cost is $2000 (200 x $10). For the 100 units, since they will not be replaced, if without this job company can actually sell them or use them to generate contribution of $15 per unit. It is obvious that company will sell ($18 is more than $15) if without this job, so by taking this job, the opportunity cost is 100 x $18 = $1800.
Total relevant cost = 2000 + 1800 = $3800.

Relevant cost for labour
Again this needs common sense. When labour is hired the relevant cost is the cost of hiring. If labour has free time to do this job then there is no incremental cost (unless overtime) so relevant cost will be $0. If labour is transferred from other job to do this job, then again opportunity cost arises as contribution from other job is lost because of this job.

Relevant cost for non-current asset
If without this job, non-current asset such as machine might be sold for money or used to generate contribution. By taking this job, opportunity cost occurred again and this time, opportunity cost is the higher of NRV or contribution from using the machine elsewhere because we want to know the maximum that we lost. Alternatively, the machine could be replaced and therefore there is a replacement cost. In such situation, replacement cost can be compared to opportunity cost identified earlier, as we want to save cost, the relevant cost will be the lower of replacement cost and opportunity cost.

With the above idea I hope the relevant cost concept is clearer to you now, as you can see there is nothing to remember. :)