The cost of skiing instead of working a part-time job is the cost of lost wages plus the costs related to skiing.
The cost of skiing instead of going to the library to study is the costs associated with skiing along with the knowledge that could of been gained by going to the library. Since the cost of going to the library cannot be measured in monetary terms, it will vary from person to person. A person who needs to study for a final to be taken on Monday may give up more than someone who is going to the library to work on a project that is not due for a few weeks.
the cost of lost wages plus the costs related to skiing.