Slitherine

It costs 40 ore to build, 2 to upkeep. Same as Scout Sentinel.

VS. that same Scout Sentinel it has 4 extra armor (32% extra damage mitigation, situational assuming enemy doesn't have Armor Penetration) and 2 extra armor penetration (this adds 2.8 dmg to 18 dmg base attack).

Sentinel though can move around to remain useful as battle lines shift. It also than gets better damage from a quest perk and later research for lasguns, so pretty soon into the game, the damage advantage swings into favor of the sentinel.

Leaving 30% damage mitigation - but being able to retreat in a pinch is better for survivability.

The Bastion really does not seem like a good trade-off.

I don't know how other players manage their economies, but in my games, the economy is always extremely tight and my power is always limited by resources far more than production time.

My production buildings are snoozing most of the time - so the fact that Bastion is insta-build, while I need to wait 6 turns for a Sentinel is usually a very marginal advantage.

I always thought Imperial Guard is a faction which would be good at turtling, but the only defensive building they get is this, and it frankly seems to suck for the cost, compared to the very first armored unit you can spam after doing your first quest.

Well it depends. Bear in mind you can build it as needed so it can be deployed wherever your tech priest is. Buildings them for the long term is possibly not cost effective but as a rapid boost to defences or as a way to harass enemy cities I find them pretty useful

I agree that in current state Bastions seems to be a bit missed idea. I would gladly pay for them the double in-game price if only they would have two things improved:
- toughness
-range

Second thing is IMO quite important, as fights in town seems to notoriously mock player with limiting range of fire (it's usually working for disadvantage of attacker, but I've recently found out many situations when my bastions were attacked from safe distance of 1 field(!) and couldn't fight back.

Currently they're ot bastions, but rather squishy outposts.

they are fantastic unit. good for tank defensive lines. they are great for base defense and limiting enemy positioning.

Actually, I would rather have them protect infantry units (like Starcrafts's Bunkers) than have their own automated defense system.
They would remedy one of the AM problems (the brittleness of its infantry).