There arent trusted ways to you to be able to make a stable income with your investment on such low time. You can try sport betting or trading some altcoins but make 0,50bt each month sound a bit hard really.
I agree with trading , you can earn profit for short till mid-term if you choose right coin you can earn 0.5 btc shortly , but you must have good knowledge and little of luck to choose the right coin to invest.