To sum it up, it increases income by 50% at a base level, and 100% if it's both "improved" (via story points) and has an Alpha Core.
Also, i've changed the impact of stability on income, which the tweet refers to. Below 5, it still works the same, but above 5, it's 100% income regardless, i.e. there's no more income bonus for having high stability. That's been a bit of a design annoyance in the past for various reasons, and makes high stability too much of a must-have. With this change, high stability is a benefit militarily, but not financially; it's nice to untangle the two and have a wider range of "acceptable" stability values.
So, basically, Commerce gives you way more income, at the expense of defenses (and, later, presumably offense as well). +100% income means that it'll be the best financial choice, likely even at 2 industries total, as long as you're able to maintain stability of 5 or more.
I forgot who pointed this out, but it sounds like player
needs this new Commerce to have the same income as today. Without it, player makes less money than he can make today without it. In other words, no-Commerce income will be nerfed, and player needs this industry - that has a massive penalty - to maintain current income... at best. Having Commerce means one less industry for exports.
Unless the babysitting problems get fixed, I think bigger defenses will be more important. The babysitting the player needs to do is insane, and gigantic fleets that can reliably automate some of it is great at removing a little of the load (player still needs to save core from pirates that refuse to stay down for more than a day, and grind rep for core worlds that keep sending expeditions).
However, it seems like Commerce will be useful after a total core kill, when there are no more babysitting problems. Got to squeeze more income from Population and Infrastructure when there is no more income from exports.
It would be nice if the new Commerce boosted more beyond income, either accessibility and/or population growth. Right now, I use Free Port primarily for population growth bonus (and maybe meeting demand once my colonies grow bigger than core worlds). More money is nice, but I gladly throw it away for total core kill to stop the babysitting insanity.