It's hard to determine sales per click as a measure of if an ad is working or not because it depends entirely on its component parts. (Individual keywords) which will vary wildly in relation to clicks per sale and go on to provide you with an overall result.
The first thing I look at is my motivation for running the ad.
If I want to get reviews and sales for rank on a new book and I price it at 99c, then rather than look at sales per click, I set an overall budget I am prepared to spend (lose) for that promotion. I say loss, because at 99c with only 35c royalty, 1 or two sales per click will likely put you into loss territory.
So to start with, royalty at your price point is all important, and after that, it is maintenance of your keywords and giving them sufficient time to collect data for the individual cost per clicks to work their way through.
Here's what we know as a fact and it's fixed. Once per week I pause any ad that reaches 70%. and especially those hidden ones that have made no sales and have used up the royalty of 1 sale in click costs. What I could do is to pause them all but one that has produced 80% of the sales over a long period of time at 37% for the cost of clicks and I'd be in the money.
Back to the original question. Sales per clicks is an individual matter, and for results to be profitable then it could be that you need to lower your bids and to increase your keywords to tweek it into profit.
Not sure if any of what I've said makes sense, but there it is