Warship construction costs
The cost of naval construction. I'm not going to go into a lot of detail here, for a lot of reasons, but the cost of naval construction in Manticore in the year 1921 has come down significantly since the beginning of the war. The numbers that are being extrapolated from the prize money Honor was awarded after Hancock Station are flawed from the outset because the price that was used to establish the value of the surrendered Havenite dreadnoughts was based on prewar costs of similar ships, and not on what it would cost to build the same vessels in 1921. (None of this is really germane to the reason people were trying to figure out the costs involved - which is to compare kumquats to pomegranates in an effort to demonstrate that prolong cannot possibly cost as much as I said it did or the Star Kingdom would bankrupt itself in the Talbott Quadrant - but it's still worth considering, I suppose, especially since people are obviously so curious about it. )
There are several reasons for the drop in construction costs, including the fact that prior to the war, and despite the naval buildup, they were still buying their vessels - and building them - using peacetime construction, inspection, procurement policies, etc.. There is a distinct difference when one begins amortizing costs over "we'll build a class of six superdreadnoughts and make lots of detail changes while they're under construction because, after all, there's time to get it right" as opposed to "we'll build a class of sixty superdreadnoughts using plans which have been frozen for production rationalization under wartime emergency conditions." [edited for clarification -Ed.] Moreover, production facilities have been enormously expanded since the outbreak of the war. Go back and look at what I've said about the growth in space stations like Hephaestus, and that doesn't even include the use of dispersed yards to further expand production capacity and help drive down costs of unit production still further. And, in addition to having expanded their basic industrial plant, the Manties have continued to look for innovative ways to speed construction times and - a huge consideration fighting a war on this scale - simultaneously drive down costs. It should also be pointed out that they (and Haven) are building warships on a scale on which no one has ever before built warships. The Solarian League Navy has a huge absolute number of warships, but it builds them relatively slowly and - for a fleet its size - in teeny-tiny handfuls. Other navies/system-defense forces which build capital ships, also tend to build them in small numbers with lengthy construction times and hence much higher sticker prices. Manticore's construction policies and practices were already much more productive than almost anyone else's, even before active hostilities broke out. Haven's were substantially less productive (and more expensive) than Manticore's, but still better than probably 85 percent of the rest of the galaxy's major navies. Since that time, these two navies have probably - no, scratch that, definitely - built more capital ships between them than the entire rest of the explored galaxy (with the possible exception of the Mesan Alignment, although I'm not saying that's the case, you understand) combined. Throw in the Grayson experience in approaching the construction of modern capital ships from an entirely fresh perspective, coupled with the freedom to adapt the best Manticoran techniques and then improve on them, and you get a situation very much like that of Great Britain versus the rest of the world in the period from, say, the late 1880s through World War I, only on steroids. British shipyards could build capital ships faster, and more economically, than any other shipyards in the world because of their combined advantages in industrial plant, experienced workforce, and experienced design force. Manticore and Grayson are very much in that category - Manticore, in particular, considering its merchant fleet on top of its huge war fleet, is the premier shipbuilder of the galaxy, people - and Haven isn't that incredibly far behind, in military terms, at least, after 15 or 20 years of warfare.
What all this means is that the construction cost of a Manticoran SD(P) is actually somewhat lower than the construction cost of a prewar Manticoran dreadnought. In addition to all of the economies of scale, increased productivity, etc., the ships actually have significantly fewer weapon mounts, require far less in the way of life support, have far, far less internal compartmentalization (since the pod well can't be compartmentalized), etc.. They do have more internal armor, especially since the practice of armoring the inside of the pod well has been adopted, but even there they probably don't have as much more as some people are assuming, since they're not building in the armored cofferdams between compartments when the compartments aren't there anymore. And, frankly, armor in the Honorverse is about as dirt cheap as any building component comes. Now, the primary weapons cost for a podnought is to be found in its missiles, not its shipboard weapon mounts, but we're talking construction here.