As of 1 April 2014, the current prescription charge is £8.20 (though certain contraception methods are prescribed free of charge).
Many students, depending on their income, may be eligible for help with prescription charges. To claim for this, you will need to fill out a form HC1. You can collect a form from your doctor or from http://www.nhsbsa.nhs.uk/HealthCosts/1128.aspx