Just having them up. It's kind of like the middle man you help the network and you get paid for it. So technically at this price you'd only get around $13 a month running one. However for every new decimal place the master nodes payout will do so too if it was $1.15 each it'd be 130. $13.15 ea it would be 1300. and so on and so forth.
Yeah it should be sufficient enough to do that. Just make sure you use Ubuntu because all other Linux distros are a pain to get up and running. Also if you plan to use wifi to run it make sure you have a good USB Wifi adapter they work like a charm.