$300 is way too low to start mining business, you need a computer which cost $300, + a Good GPU , lets just say you buy an RX480 at $300
it will get you 0.5 eth, thats just $50 at top,
dont forget you need a system to support gpu, electricity bills, internet costing, that is way too much expansive then $300, you atleast need $1500 to get a small rig and then start mining otherwise use your $300 to invest in altcoins, and make profits from there, once you go over $1500 think about start mining