Going to the dentist has the benefit of continued dental health, associated with a higher quality of life and additionally with physical attractiveness. The costs of these services are the actual direct financial costs of paying the dentist as well as and pain and suffering associated with the work that is performed as well as the time which must be dedicated to actually being at the dental clinic.
Completing homework offers the benefit of education and good grades, both of which may be associated with a higher income as a working adult. One may also receive the benefit of self-satisfaction that comes from completing a necessary task. The cost of the action is the time required to complete it, as well as any ancillary costs associated with the work such as pencils, paper, and the cost of books and/or an internet connection.
Getting an extra hour of sleep may provide the benefit of being well-rested, but this depends on the amount of sleep one would have been getting otherwise. While getting enough sleep is associated with increased physical and mental health, it is possible to sleep too much resulting in grogginess and a decreased mental state. As such, the sleep itself may be either a benefit or a cost. Additionally, the hour itself has value so this action bears the cost of the loss of time that could have been used for other activities.