The answer partly depends on the context of a particular question.
However in general the production cost is the cost of actually making the product. There are lots of other costs that are likely to occur (such as R&D and selling and delivery costs etc), but the cost of actually making each unit is the material, labour, and variable overheads in the factory on that one unit.
R&D may well have been necessary but they will only occur once – production may go on for many years.