Maybe, in an effort to approach this from a different angle and it make clear, it would be best to deconstruct Slums:
Once per turn, when you pay the trash cost of an accessed card, remove that card from the game instead of trashing it.
So, going through the order, we have:
- Once per turn (irrelevant to the discussion, but only occurs once)
- when you pay the trash cost of an accessed card (conditional ability)
- remove that card from the game instead of trashing it (replacement effect)
We can’t argue against what 1, 2, and 3 are. There is an overwhelming amount of evidence in the game and OFAQ that supports the model that this is a conditional ability that procs a replacement effect.
The first thing of importance here is that the replacement occurs when the card is trashed (IE: the exact time that CtM is fired). This portion is not up for debate.
Furthermore, as a lemma, we know that if you do a Sneakdoor run on Archives which also has ST on it, ST’s ability is triggered, and the fact that Sneakdoor occurred first (because constant abilities fire before conditional) does not change that. ST resolves after Sneakdoor’s replacement, and, thus, you cannot go back later and grab ST money.
And, really, no matter what model you use of the steps for “trashing a card”, it has to be the same interaction here. Runner-side effects always happen before corp-side effects on the runners turn (99% of time when runs are happening).
If you use the model put forward by @jakodrako, where you have:
- Pay trash costs.
- Trash.
Then what Slums is doing is setting up a constant ability that will result in the replacement effect happening when the “trashing” step comes. The constant ability will fire before CtM.
Even if you used some weird model where you have:
- Pay trash costs and trash (simultaneously).
So Slums isn’t setting up a constant ability within its conditional (it’s just directly replacing), then it would still resolve before CtM, as it is a runner-side effect on what is likely the runner’s turn.
In both cases, all effects that occur in response to an event (in this case, trashing) are considered triggered, regardless of successful resolution (from our lemma).
Where I’m going with this is; from my perspective, I cannot come up with a game model that both supports the rulings being made on Slums, and also coheres with the rest of Netrunner.
I hope this clarifies a little. If anyone feels like there’s a valid model that supports these Slums/CtM rulings and doesn’t break Netrunner as a whole, feel free to mention it. Because I haven’t seen it yet.