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
100-200$ will barely get you a rig with a ten year old graphics card, never mind a GPU for mining. RX 470s usually cost around 130 bucks and combined with a cheap LGA1150 and celeron G1840/ Pentium processor as well as 2-4G of ram, you've got yourself a very small rig which can be easily upgraded.
Yes, R9 300s and R7 300s still work well, but I'd suggest going with the newer RX series or waiting for Vega. It'll be making you profit longer.