if you don't want to spend much, but still want to learn something, and return your investment at some point, you will need to spend at least 100-200$ on a used GPU, MBO and CPU, hook it all up, choose a coin to mine, Ether for instance, later you can trade it on Poloniex for instance for Bitcoin, or simply sell it as it is on Coinbase or the like

ROI would come after a year or so, maybe less, depends what GPU you choose and the Price and difficulty progress of the coin