For most citizens, the costs would be tangible and the benefits would be intangible.
The major cost of completing a term of national service is connected to what the person could have been doing instead of national service. Many people would be able to make a great deal more money and get a start on a career if they were not required to do national service. By serving, they are forced to give up the pay they could have received during their service.
The benefits of national service would be psychological or emotional. The most likely benefit would be that people would feel good about themselves for having served their country. They could feel as if they had "done their part" to make their country a better place.
People who do national service, then, tend to have tangible costs from lost wages and intangible benefits from a feeling of having helped their country.