First off, I gotta get this off my chest: To me, I tend to not get so wrapped up in Tapastic's ad revenue. Sure I track it and monitor it, but what I don't get wrapped up in is how it's not constant and consistent. That's really just the nature of ad revenue. It's not reliable and it fluctuates daily. I see ad revenue as more of extra money and not sustainable moneys that Tapastic was awesome enough to offer to us. As comic artists it's important to constantly seek more revenue streams than focusing on one. [ie. book sales, conventions, commissions, freelance, day jobs]
Phew, Ok, I feel better now that I've said that.
Now to answer your reply, 20 cents per 600 ad impression is actually the same as what I said above with my graphs. 600 divided by 34 equals about 18 cents (close enough to 20 cents). 34 ad impressions to earn 1 cent is this month's ratio. We'll have to wait and see what next month's ratio will be since it does changes as my chart shows.
Last year's rate was different from this years rates as well.
Last year's rate was about 13-15 impressions to earn 1 cent.
And there's new features relating to ads and ad revenue launching this month that can have an impact on this. Once it balances back out it might be for the good, but till it balances we might have to deal with some lower earning days/weeks/months.