I'm self teaching AQA Biology (new spec) , AQA Government&Politics and Edexcel History (new spec). I've been looking at exam centres to do the exams in 2017 , however I'm confused about how much it will cost me. For example Campbell Harris says £60 ,£70 & £120 per unit but I'm not sure how many units my subjects have , so I would like some help to figure this out and if you know any cheaper alternatives please tell me. Thank you