Stanford students can receive course credit for their Aikido practice.
Register for "Athletic 3M Aikido". In order to receive credit you will need to attend at least 20 hours, which works out to about twice per week.
There is no minimum attendance requirement if you are not attending for credit, but dues of $25/quarter are expected in either case.
Attendance of once per week should be a personal minimum, and three times per week is generally accepted as a good level in most martial arts, in order to progress. Dues for non-undergrads can be found here.
There is no charge to try some classes, but we do ask that you fill out a waiver. If you decide to attend regularly, you will want to purchase a gi (practice uniform), which will cost roughly $30 - $50.