College newspaper and radio stations often have special advertising or underwriting rates for registered student organizations. If not, form a coalition with other groups to negotiate a reduction. Contact the respective advertising representatives in advance for information. Some large universities, with several bus routes, allow organizations to place advertisements on the sides of the buses. Contact your campus' office of transportation and see what you can work out.
Organizations can also buy airtime from local media outlets for coverage.
Better yet, if you co-sponsor with your union programming council, they may foot the cost of the ads. If a guest speaker has free time before an appearance, contact the college and local radio stations in advance and schedule on-air interviews. This is a terrific way to raise interest in a speaker while working in several promotions for the event. Also, hosts of campus talk shows, both radio and television, are always looking for new topics and guests. If your event doesn't include a guest speaker but has a theme that can tie in to an interview, debate or call-in discussion, give the radio station a ring.