@Zelnik
No, the premium you pay is not worth it. All the things you mention have been accounted for and the Bastion still falls short. The Ld may start high, but falls away due to attrition. So it gets worse the more it's needed. This is pretty much self regulating, being neither better nor worse than normal leadership, just different. Yes, being able to ignore BMs for movement is a bonus, but being forced to disengage when crippled is a negative. Again, this stuff balances itself out for the most part. I think taken all together that the Demiurg rules come out slightly down. That is, they serve to reduce the cost of the ship they're applied to. Apart from these rules they have multi-arc torpedoes, including backwards, as well as a bit of in-game ordnance versatility (from the Bastion). The most flavourful item though is the cutting beam. Typically this weapon does not-much. Sometimes it's really pretty good. I have no qualms about it being unreliable, it's just that it should be noted as such and not overestimated.
However, the biggest issue I see with your contention that the "premium" you pay is "worth it" is that this premium differs according to the ship. I think that I've shown that the differences in the value between the Citadel and Bastion and the Bastion and Stronghold are inconsistent with their price differences. So if you purchase a Citadel or a Stronghold for your fleet you're paying less of a "premium" than if you purchase a Bastion.
@Zhukov
Changing the point value when in a dedicated fleet or against Orks is untenable. It would make taking Demiurg to tournaments as allies would be impossible. Instead the ships give up considerably more VPs in those situations. This is fine, particularly as the extra resilience is only fair to middling (1 hit leeway on the Citadel, 2 on the Bastion, 3 on the Stronghold).
Rarity by fluff should have no effect on the price either.
As for compositional advantages I think I disagree here. While a lot of races can use them the most likely to is the Tau. The Tau typically want to take ships that allow their better ships to be taken. The Demiurg vessels will not allow a Kor'vattra player to take more Heroes nor would it count towards the minimums allowed for a Kor'or'vesh player to take Custodians. So a sacrifice is made here to take Demiurg vessels (particularly as the Hero is very strong competition for a Bastion). Similarly, in other fleets Demiurg ships don't count towards heavy cruisers, grand cruisers or battleships. So the ability for other fleets to take the Demiurg is pretty much balanced by the compositional restrictions.
In a pure Demiurg fleet again compositional restrictions work against the Demiurg. They have 3 ships and 2 models. If you're like me and don't accept the use of the Bastion model for the Citadel (meaning a scratchbuild) then this makes for a very restrictive fleet list. The Demiurg should really get a points break for this harsh compositional restriction. Particularly as you'd need 2 Bastions to field a Stronghold, which makes for a strange '1 Stronghold per 860 pts' limit. It is really quite hard to make a 1k Demiurg fleet too.
Now a note as to battlefield usage. It has to be fairly obvious that the Demiurg ship designs are quite conflicted. This isn't really a bad thing, since they have a lot of firepower and they're really just mining ships, but the fact remains that they're not efficient.
So, let's say you head straight at the enemy in order to try to maximise your cutting beam efficacy. Now you're wasting the range on your lances, which is wasted points. If you go abeam (perpendicular to approaching ships) then you're wasting prow weaponry and armour and off-side weaponry and the short range of the WBs is unhelpful. Of course, this is not what you would do. You'd approach obliquely, getting a target in the prow and a side. This will allow you to make use of your long range lances, prow WBs and torps. So, at 60cm you get 2 lances and 4 torps (Armageddon = 2L + 6 torps). At 45cm the Bastion gets 2L+8WB+4T while an Armageddon gets 4L+6WB+6T. At 30cm the Bastion gets 2L+14WB+4T vs 4L+6WB+6T. So far the Armageddon has won every single range bracket while being used the same way. If you get a target in the third arc it will likely be at 0-45cm meaning they will both perform the same for the most part, with the Armageddon edging out in the rare 30-45cm offside engagement.
The Bastions advantage lies in the ordnance versatility, raising of fleet AC limit and those occasions where the cutting beam comes into play. It loses out in range and focus. These ships are pretty close in capability, with the Bastion harder to use to effect and a bit more gimmicky. I don't see why the Bastion costs 20 pts more. For neither composition reasons, special rule reasons or rarity reasons.
TL;DR - point for point the Bastion performs poorly compared to the Citadel and Stronghold (internal balance) as well as against its nearest analogue the Armageddon (external balance). It's compositional restrictions make it just as bad as a battlecruiser in allied fleets and its cost makes it a nightmare to construct a 1k fleet in a pure Demiurg list.