This from the Morbidity and Mortality Weekly Report (CDC)
http://www.cdc.gov/mmwr/preview/mmwrhtml/rr4907a2.htm
Break it down like this: You're in a high risk category, (you white freshman dorm-dweller with radiant heat and a recent cold). The incidence in college students is low (1 per 100,000) but you have to weigh a $90 shot (known cost) vs. the somewhat low risk of catching a disease that can easily kill you. Get that part? Kill you. So for $90 you eliminate a possible cause of death. To me, that's cheap.