The balance for these missiles feels really strange.
Since the normal micro missiles need carbon, you can't make them on self sufficient space platforms. For anything other than asteroids, you certainly want homing though.
But homing currently costs a whole processing unit per missile! At that point, it's much easier to just build more normal-sized missiles and more turrets (or even deploy destroyer capsules).
Explosive micromissiles also cost way more explosives per damage than normal explosive missiles (especially since their AoE is much smaller - normal explosive missiles cover 3.5x the area, in addition of dealing 3 times the damage!), in addition to lacking homing.
Kinetic and arc missiles sound very fun, but are similarly heavily overpriced.

Perhaps a more balanced solution would be to replace the carbon fiber with steel (possible to make in space, though requiring either a lot more furnaces, or an additional foundry), allow the addition of homing with a single red chip per magazine (since normal missiles don't even take green chips now, this would still be a large step up), and also make arc/kinetic missile upgrade apply to whole magazines (or at least large batches). 2-3 explosives for 20 explosive missiles, 1 tungsten plate for 20 kinetic missiles (cut into smaller shards, since a micro missile couldn't ever carry a whole tungsten plate), and maybe 1-3 capacitors for 20 arc missiles (since they can stun, they could be quite useful - but on the other hand, at that point in the game you could just deploy arc turrets instead, or even artillery...) could also make them economically usable at least in some situations, instead of being something you'd only ever produce for the fun/challenge of it.

Well, some of it is kinda intentional - remember that these have times 10 times the fire rate, extended range and high speed, making the overall dps higher than regular ones. So while basic version is cheap, upgraded ones cost quite a lot since, I didn't want them out competing basically every other turret / missile, and they require materials from basically every other planet.
...I guess maybe way too lot.
But factory must grow for a reason, right?
And you don't actually need homing that much - a huge biter swarm or nest is hard to miss anyway.

But I guess it's fair to add a cheaper mode in settings for someone who wants them earlier in the game - cutting costs like by an order of magnitude or so.

... By the way, have you ever loaded a spidertron with a mix of these? If not you are missing the fun

Added an option to make recipes cheaper in the new update

Chiming in here to say that the concept of the mod appeals to me but it doesn't feel usable in the way I was hoping. My mental image was that it would just be like the standard rockets but more projectiles and loads more fun to look at. Like said above the carbon fiber not being in-situ attainable feels bad, and the rocket fuel is doable but raises the comparative complexity of making the micromissiles by 2-3x with the coal liquefaction requirements.

I think the mod makes a lot of sense as specialty weapons to equip on spidertrons or other modded rocket-launching vehicles, but on space platforms (where the immediate appeal is strongest) it feels exceedingly difficult to make happen.

Well
It's probably more meant for combat vehicles and defending certain areas rather than space platforms, as you correctly said, in-situ costs of them are rather high.
So you are right about them not that great for space platforms, it's just not their main use.
Go download Lex's Aircraft, set up patrols, and shower the biters and pentapods with missiles from air instead, way more fun.

After multiple complaints, I've added an alternate recipe for micromissiles using LSD instead of carbon fiber. Unlike fiber, you CAN produce it in space sustainably, and will certainly have enough on the planet as it's needed for rockets and has productivity research.