It depends on what you are willing to do. I run the apps (at least 60 SB per day) and run nCraves (another 25-100/day, depending on how high the payouts are, and how much I am home). It is mostly 3SB ones now, so I figure on about 9SB an hour, for 6+ clicks an hour (or 15 if I don't hit to turn discovery mode on, and its laggy so I keep forgetting again and again, sigh). If I am home anyway and don't have a lot going on, just glancing at the computer every 20 mins is not a big deal to me.
Of course, that's only about $45 a month, at an earning rate of 120/day, assuming that allows you to make 1st goal daily. But, I also get shop and earn from churning gc on ebay, and my actual average is closer to 180 or 200 (and was 300-400 in Sept-Dec); I haven't had a month below $75 since auto nCraves started working for me in June. I rarely do anything that requires me to spend money (other than S&E for things I was already going to buy, when swagbucks has the best payout), although I did do the Kinsa offer in January while I was at Disney

So, $50/mo is pretty easy, and $100 is reachable with just a little more effort.