These sales work well to generate nice little bursts of activity. They make money. But their overall contribution to my bottom line looks pretty slight overall.
I downloaded a CSV from PayPal, which I use (despite obvious misgivings) for the majority of my info product business. I then filtered it only for sales of my book, and did a little math. The code's in a gist. The numbers look like this:
This means the discounted copies made up about 16% of the total copies sold and 10.6% of the total income from the product. I did this a couple days ago, so I've sold a few more copies since then, and I've sold a few copies outside of PayPal as well, but these numbers are good enough to draw some basic conclusions from.
However, I also haven't measured how these little sales affect traffic or attention, and I think that would be harder to measure, but my hunch is sales have a benefit there. Despite the fact that basically 90% of my business comes from somewhere else, I would still recommend sales anyway, because of these harder-to-measure benefits. They also make it easy to drive short-term spikes in your product income, which is good for cash flow.