"A total number of 1400 soldiers were subjected to prophylactic regimes with either doxycycline 100 mg/day (n=986) or mefloquine 250 mg/week (n=414). Prophylaxis lasted about 12 weeks. The side effects and compliances were investigated by questionnaires. All soldiers were monitored up to 6 months after returning home. RESULTS: No malaria case was observed and there was no severe side effect in either group." - J Infect. 2005 Oct;51(3):253-8

Doxycycline = £3 ($5) per gram (British National Formulary September 2005)

Therefore, £110 ($190) protects 1 person with malaria for a year, or 30 pence (50 cents) a day.