Cheaper, more expensive... Dollars are a really bad way to measure price. Money in general is...
Work hours or work years are slightly better. Or other resources. How many years would it take for a single worker to build a nuke, translates roughly to how many workers are needed to build a nuke in a year... or how many in a month. Or week.
How many people are there in GTVA, without Terra and Vasuda Prime? And after the Great War, GTI Rebellion, NTF insurgency and second Shivan Incursion? I'm guessing on scale of hundreds of millions. Possibly somewhere between a billion and two, but I'd say that's stretching it a bit. How many people you need to get the materials (mining, refining, transportation), do maintenance on mining and manufacturing facilities and assemble the warheads (using machinery of course, but still)? The worth of the weapons is at least the living expenses [salaries] of all these people.
Granted, the GTVA seems to be mass producing kiloton grade nuclear warheads (tempests lol) and bigger ones, so they probably do get off somewhat cheaper than on current day... but still, the effort and people involved would still cost a lot. The rarity of the raw materials is still an issue, whereas with space ships you have a lot of practically pure iron asteroids hanging around probably in most solar systems. Getting the reactors and all the systems working, same logic applies - basically the availability of materials is what defines the "price" of a mass produced ship, weapon or other gimmick.
So I'm not saying that things in general won't get cheaper. Remember I used the analogy of abundant resource, iron (steel) being the reason why cars have been able to become so much more available than they used to be?
Of course, you could argue that since GTVA seems to have abundant* amount of nukes and other munitions at their disposal, they
must be cheap and they
must have abundant resources on making them... but I'm just saying, if we assume that the bombs like Harbinger use similar technology as current day hydrogen warheads, their manufacturing process would be roughly as resource-intensive as in current day. Of course if they use the same magic that moves the ships around, I have no argument against that...
And talking about bombs and taking out Colossus or destroyers... the beams are still more cost-effective to use than bombs, as long as you have to use capital ships anyway. Colossus-style ships are just stupid but that's not the point in this discussion... spending 15 years on one warship is simply staggering idiocy in my eyes but as it's mainly a plot device anyway (we don't really see much of Colossus pwning the NTF) I don't really care either.
It goes like this - the enemy [shivans] have cap ships that are difficult to destroy. They can be destroyed with either bombs or beams (grossly simplified but essentially how the matter is). To destroy them with bombs, you need bombers and fighters to protect bombers. To use bombers effectively, you need carriers (destroyers) to haul their ass around. So you end up with cap ships of their own, and we've already established that cap ships using bombs is not as effective, sensible and cool as using beams (space and weight constrictions of hauling ship-to-ship munitions around instead of more bombers that could be used to deliver the munitions more effectively, and beam cannons offer more total firepower since their only ammo limitation seems to be energy from the reactors and heat dissipation speed...) - so the cap ships use beams rather than physical munitions.
As long as the tactics works and the fighters and bombers help keep the cap ship alive, it doesn't take long before using beams becomes more cost effective than using just bombs to destroy things. Of course, the way Command wastes ships that rarely happens, but that's besides the point...