You are not going to establish any of this until you start assigning some numbers. Put a pricetag on how much it costs to manufacturea a telescope of a given size, resolution, field of view, ect. and we can get a discussion going. Intelligence gathering and their countermeasures don't develop in a vacuum, and that's exactly what you're trying to do.
Determining how much telescopes will cost is going to depend on knowing a lot of stuff about the civilization's technology level and logistics that we don't know. The best thing I can do with what little we have is to try to estimate how much material sensor nets of different size would consume.
OK, I'll try a hypothetical test case. Let's use GMT's calculation based on modern telescopes being able to get an image of Pluto in a one second exposure. This gives you an effective sensor range of roughly 1 AU to detect a 4 GW light in a 1 second exposure. Going by my earlier calculations for decoys, this is sufficient to detect everything more energetic than HB-fusion rockets, but not low-power NERVA or relatively low thrust (1 m/s^2) chemical rockets.
GMT also quantified earlier that using a modern wide field telescope as a base we might calculate 400 telescopes to watch the entire celestial sphere. Obviously, for a decent sensor grid you will need several times this much.
The most efficient shape to give complete coverage of that 2 AU sphere is probably a rosette of 4 telescopes orbiting in the plane of the solar system, with two statites "hovering" over the north and south pole of the sun. That gives us 6X400 = 2400 telescopes. If we assume statites are two problematic and make the polar sattelites an orbital rosette that gives us 8X400 = 3200 telescopes.
As we go outward, spherical geometry suggests the size of our rosettes must octuple each time we double the radius of the surveyed area. To watch 4 AU we must have 25,600. To watch 8 AU we must have 204,800. To watch 16 AU we must have 1,638,400. And to watch 32 AU we must have 13 million.
Now, to determine how much material is required. Let's assume each individual telescope has a mass of 10 metric tons, a little less than Hubble. That means the entire array consumes 130 million tons of material.
Now, let's say around .1% of the material a typical asteroid is useful for building telescopes like this. This array would therefore require the deconstruction of a planetoid with a mass of 1.3 X 10^11 tons. Now, a 1 km diameter asteroid might have a mass of around 2 billion tons (ref
). Applying spherical geometry, our array would require the deconstruction of a single spherical 4 km diameter asteroid.
Assuming the average warship masses 1000 tons, this represents an opportunity cost of 130,000 warships.
Realistically, a really effective sensor net would need to be at least several times denser, and therefore consume several times as much material. You could avoid being seen by this thing by using fraction of a second burns, for instance, or by using a number of low thrust, low Isp rockets.
On a strict materials basis, this network should be readily feasible. There are probably more than a million asteroids in the 1 km range (ref
), so we have barely scratched the surface of the solar system's resources. Of course, this does not count the difficulty of turning that material into telescopes, which will vary enormously depending on the technology and industrial techniques available.
Conclusion: a mature spacefaring civilization which extensively exploits space resources and has an economy that dwarfs our own would probably find this worthwhile. One that's just starting out ... probably wouldn't.
An alternate, probably more productive approach might be to try to station sensor platforms close to enemy bases to watch them, while maintaining only a bare minimum of sensor coverage elsewhere to track any burns enemy ships make in deep space. This, of course, will probably use up much less material, but will require a delicate balancing act. Your observation platforms must be able to get close enough to enemy bases to observe them in some detail, while being small and inconspicuous enough not to be detected by the enemy.
It's worth pointing out something here: you've been saying an observation platform may not be as stealthy as a warship because it will need to be a monster telescope. With this strategy you will not need a monster scope. You will need a scope capable of keeping an eye on an enemy base from relatively close range; a few hundred thousand to a few million km away.
Of course, this strategy has its own downsides. Like I said, it requires you to be able to sneak the platforms into range of enemy bases, which is going to be tricky, though it should still be considerably easier than trying to sneak around with warships. It also requires you to know where all the enemy bases are, knowledge you may not have depending on the situation. And there might well be some delay between the time an enemy base goes online and the time you can get a covert observation platform out there.