To add to what people are saying about ad revenue being the only source of income, you wouldn't really want that, to be perfectly honest. Ad revenue is a constantly fluctuating income source - there are many factors that determine how much money you make, which include, but are not limited to:
- Your average viewcount
- The average CPM
- The value of the ads that are being put up on your website
- In terms of places like Project Wonderful, the average bidding on ad space
- Users who are using desktop version vs. app (in the case of Tapastic specifically)
- Users who are using desktop version + ad block (obviously not gonna get ad impressions from these guys)
All of these factors are subject to change at any time, for better or worse, so if you depend solely on ad revenue, you're taking a pretty big risk and basically gambling your entire livelihood on how the numbers decide to stack up. When you do commissions, sell books, etc. these you can tend to control a little more in your favor. Though you will have times you don't sell as many books as you had hoped, or you don't make your quota in commissions for the month, there are still things you can do to combat these things (such as going out and looking for more work, conventions to attend, etc. or do special commission events to get some more eyes on your work) but with Ad Rev, there's really no way to control your outcome, aside from just updating as often as you can (and even then, that's very, very little control).
So yeah, while some people are able to make a lot of money off ad rev, it's definitely not a dependable source of income, and it would be in most people's best interest to not be making their only income off of Tapastic ad revenue alone.