I was curious about this, so I looked up some stuff about it.
KSP costs US 40 dollars at the store, US 39.99 at steam. Steam takes a minimum of %30 from every sale, which means (assuming $40 for KSP) that they get $12 per KSP sale. So the devs get $28 per sale. However, that's not the whole story.
Steam hosts servers that SQUAD doesn't have to host. They also provide an immeasurable amount of exposure.
Taking @technicalfool's ratio of 100tb/month via amazon, to $9779.96/month, and rounding down to 9000/month, we get a ratio of 1/90 tb/$/month.
Next we have the problem of copies bought per month. I'll take a total stab in the dark, and say that 3 million copies have been sold. About one year ago, there were 1 million total copies sold on steam. So let's say that, in the past year, another 800000 copies have sold on steam. KSP is about 1GB per game, to that's 800 tb per year., which equals ~66.66 tb/month. That's $6000 a month.
This means that the devs get $1,866,666 a month off of steam sales alone? That's almost certainly not right. But if it is, then the cost of hosting servers is negligible, and it's matter of whether the exposure outweighs the %30 cut.
This is not meant to refute your post, Red Iron Crown. I am curious whether different venues provide different profits.