You might be over-thinking this issue. The consensus here is that most dampening material for PC cases is not a particularly good investment (except weight loading side panels). There simply isn't enough space for this to have a significant effect and there are still loads of open paths through which sounds can escape (the tini-tiniest gap makes a huge difference).
If you just want to block off a direct air-path, I'd simply use closed-cell foam; the stuff that is frequently used to protect equipment during shipping.
I've read extensive reviews of precut foam kits and that's the conclusion I came to too. Which is why I was looking for cheaper alternatives. I think I might have some closed cell laying around so I'll give it a go. The case I'll be using is a Fractal Design XL R2 which already has bitumen noise dampening on the side panels, which seems to be pretty effective at weight loading but weight loading isn't a goal for the foam. It's purpose will be to block the 5.25" bays (and a few other minor areas) just to eliminate a route for sound to escape unimpeded. It should also help with dust as Fractal basically left the 5.25" bay on the XL R2 (and even the new R5) unfiltered due to the way the front bezel is designed.
That's a novel approach... but you could also go to your local foam shop and see what they have in their scraps bin. I went to my local foam/mattress store & got a huge bag (at least a meter square) for $30 (iirc) some years ago. Way too much it seems, as 75% of it is still there, to my wife's displeasure. I used bits of it for all kinds of purposes -- including corner bass traps!
Anyway, the stuff you link looks cheap enough, but it does need to get shipped to you, right? That will certainly add to the cost. Don't worry about whether it's "acoustic foam" -- that's mostly marketingspeak. All foam eventually flakes apart btw. Just a question of how long. And all foam is cuttable -- I use a long finely serrated breadknife when cutting big pieces, or a utility knife with fresh blade fully extended.
That's a great idea! Mattress foam would have to tick a lot of the same boxes (you wouldn't want it catching fire easily or dropping dust). I'll make some enquiries... I need a new mattress anyway, so I might be able to get the offcuts for free.
I don't live in a particularly urban area so (there are no foam shops even close to me and) the delivery cost on the stuff I linked isn't actually too
steep. It'd certainly work out cheaper by a long way than any "computer noise dampening kits". Now that you've mentioned mattress foam though, the stuff I linked is definitely a second resort.
Thanks for the advice MikeC and Cistron.