I'm not an expert on South African carrier economics but decent to good Android phones do exist in the 70-100 dollar range on prepaid carriers like MetroPCS and Cricket. I'm pretty sure that means a $50 USD android/other smart phone (maybe Firefox OS) wouldn't be too hard to produce. I'm under the assumption that these carriers do provide a small subsidy (even though they're contract free) which may or may not make sense for your country's carriers.