Additionally, if you're doing ads on Google, there are some weird interactions here that Econ 101 would not predict. For example, although Google describes AdWords as an auction, it is really "auction-like": the highest bid doesn't necessarily win. One factor which can cause lower bids to beat higher bids is if the lower bids have account history. You get account history by winning the auction lots of times.
If you're scratching your head thinking "How does that matter?", consider a software market where one vendor has a product priced at $30 and another has a competing product priced at $22. Their conversion rates and CPCs might very well be similar on day one, resulting in a cost-per-action of $25. One vendor might choose to participate in the auction while the other does not. As the participating vendor's campaign picks up positive history, Google may cause their CPC to decline due to improved account history, perhaps resulting in a CPA closer to $12 ~ $15. Vendor #2, seeing this, might think "Darn! I'd sure love to advertise if it cost $12 ~ $15", and then turn on their ad campaign... only to find that it still costs them $25. (In some cases, it could actually cost more as a result of their competitor's success.)
The above example is somewhat simplified. AdWords can get complex fractal-style.