I've been caught exactly like that, but on a different site (etsy.com) for exactly the same circumstances.
I 'believe' that the CPC/CPM model is designed to escalate bids regardless of 'competition', in other words, you're competing with an algorithm, not another bidder. I'm of the strong opinion that these bidding systems are inherently fraudulent and designed to hoover vast sums from advertisers. You get just enough response to make it viable, occasionally slipping into a profitable loop which eventually gets caught up and drops you out. Sure, you get clicks, and sales so there's nothing wrong with the principle, just the algorithm. It's like a slot machine, you'll always end up putting more in than you take out. #justmyopinion