Wow man that's a very good return. You must also get natural traffic, and is the $100 the only expense in marketing? Whichever case it's still excellent. You should spend more from your profit and raise your revenue such as spending $500 and earn 5 times the amount you're earning now.
I don't incur much in hosting costs and don't pay for advertising, so i've never really look at my actual profit..i just know i make some extra saving money I don't rely on it so never factored it up, I will tally it up and return here!
A rough estimate is about $480/month...sometimes much higher if i am promoting affiliate signups like webcams.com or pu*sycash I made $700 in a month from pcash, when they were doing $150 per signup. I don't promote them anymore though cause I got converted down. And webcams is just when I feel like messing with it.