Enjoying the concept a lot (just K2, without also SE). I did eventually notice in marathon base Factorio a couple of buildings with different costs that don't mirror in this expensive universe: the steam engine and the electric mining drill. They could be more expensive here? Eg somewhat parallel to base Factorio cost jumps:

Steam engines: 50 plate + 10 gears + 5 pipe (currently 11 + 8 + 5 like non-expensive K2)

Mining drills: 8 beams + 12 gears + 2 automation core (currently 2 + 10 + 2 like non-expensive K2)

(There also doesn't seem to be an Expensive Universe cost jump for rare metals ... but perhaps rare metal plates could get a x2 on ore and smelt time? like steel? I was thinking about expensive electric miner 2s.)

Yes, thank you, those were minor oversights (and a typo) which didn't take place correctly.
For the special K2 resources, I'll take a better look on how to make it more expensive. Maybe I did it for K2SE but not for K2 alone or it didn't come out as expected.

Always open to feedbacks & ideas on this, if you feel something can be made more expensive, I'll get it out for next release within the day.

My general rule of thumb is making intermediates more expensive, so it translates across the board

I have still been wondering about this a bit, and yeah, tweaking intermediates seems feels right & aligns with how base Factorio mkes things expensive. With that in mind, a couple of other reflections (mostly) regarding the K2-specific resources. Many already seem fine, as in, appropriately more costly.

*Glass & silicon: don't need to be individually more expensive, the electronic components entail a cost jump.

*Rocket fuel, atmospheric processing, other fluid/gas chemistry: there's a lot of new gasses but I don't think any costs need to change. Base Factorio doesn't really make petrochem recipes more expensive, just makes the downsteam items more costly. Eg blue chips taking more acid, which the expensive universe also does. More acid / chlorine is already needed for the extra uranium / rare ores, I don't think anything in the K2 specific chemistry needs more adjustment.

*Powergrid / fuels: fuel recipes don't seem more expensive in vanilla, the player already has to build more grid to power more of everything else. So processes into eg tritium or fuel don't need to change in the expensive universe either.

*Wood & biomass: These currently don't change in expensive universe vs normal K2, and base Factorio doesn't really have an analogy. Maybe these buildings could just make less? Maybe half the output number, or else double the cycle length? Not a huge thing.

*Uranium: Usually this is a power source but in K2 is a science ingredient. Expensive universe centrifuges eat more uranium ore than normal, that's good way to do it. It's fine.

*Lithium: The lithium-sulfur batteries get more expensive, affecting advanced and singularity tech cards, but not matter tech cards. It's good, I don't think everything needs to get worse everywhere.

*Immersite: The expensive universe is more expensive, but I'm not sure if just needing more quarries is as interesting as it could be. It doesn't subtly change ratios like expensive green or red chips do. What if crushing raw immersite gave 4 stone instead of 3? Not a big deal.

*Matter: It's either for antimatter powergrid, which doesn't need to change because powergrid, or else whatever you're converting into will encounter expensiveness later. I wanted to mention I considered it, but it does already seem fine.

So, having tried to be comprehensive, I'm concluding that pretty much everything is already good. Maybe a note for wood or biomass, but they're not a big deal.

I agree that vanilla's expensive mode, other than being "harder", it has the nice touch of changing the ratios of your builds which is also very refreshing as game experience. And this aspect I may have overlooked a bit just focusing on increasing the costs in such a balanced way, that the ratios may have not changed at all, all things considered. Will have a deeper look into it

Yes and the ratio changes are subtle with consequences, and expensive universe does that pretty well already. Plastic in LDS, coils in red & green chips, like base does. Rare metals boost in blue chips. Glass & silicon bumps stone use in red chips in a different proportion to module upgrades. Nitric acid bumps mineralised water use & also changes ratios for AI cores.

It doesn't need to feel like a recipe randomiser or another overhaul mod beyond K2, just more expensive & a couple of small twists. I kept thinking about it, but even these other tweaks might be to much:

*I was thinking about immersite: maybe the immersite crushing could also kick out a rock? 6 raw -> 3 powder + 3 sand + 1.5 rocks? Which violates the "only more of the same input/outputs" guide, but forces a slightly different treatment, but not very much.

*Maybe the coke recipe could be 12 (instead of 6) coal + 6 wood -> 6 coke? It also makes coke much worse as a fuel, which is fine.

*Maybe each of the rocket fuels could use 1 steel plate instead of 1 iron? Or the extra spicy version (maybe too spicy): 1 empty barrel

*Maybe knock 10% or 20% off the matter output of all matter generating recipes? To not be a direct conversion from one material to another. Not a big change but might make players reevaluate it.

But overall I don't think the mod wants to be expensive just for the sake of expensiveness. Players can still mess with patch frequency / technology multipliers in map settings. I think the goal is to match the cost/increase of "expensive" recipes in base, and maybe also aim to not go past that either. And I think it's already very close.