There used to be a sub-telco called Genie in the UK (powered by O2/One2One) when I was in school, they offered unlimited texts on a pre-paid sim card tariff for £10 a month. I used to tell them around my school for £15 per sim card, I learnt a lot about supply and demand... eventually, I became known in my area and started to sell them to students in other schools, this kept me in pocket money for a year before they dissolved the sub-telco. It wasn't much but it was enough to make me understand some principles of business.