Starships: The Planetoid Quandary, Or Are You Getting Your Money's Worth?
Let's get this out of the way first, are buffered planetoid's worth it? No, because their concept was always a military one, and that extra level of protection was added on to the biggest possible armour factor that was allowed for that technological level, making them rather invulnerabilish, and that raison d'etre has been removed with the current rules. The extra fifteen percent difference between it and the normal variant allows more equipment to be installed, without sacrificing protection.
They're great for system defence or just paddling around local space, the issue becomes less clear if you install a jump drive.
In terms of hull costs, you have to figure that an equivalent normal starship will be ten times more expensive, that includes shrinking it back by one fifth because of the twenty percent waste space. So any savings will have to come out of that ninety percent, because you are going to have to scale up engineering by a quarter to get the same expected performance from a planetoid.
Does that include armour plating? In my opinion no, because the inherent factor two armouring would ensure that added on armour would only be needed to cover the internal leftover volume, so it would be based on eighty percent of the planetoid, not the full hundred.
You could, of course, build an equivalent ship without gravitational plating or inertial compensation; whereas I have my doubts on the usefulness of inertial compensation for most forms of commercial traffic, that's not going to fly for military craft, and in Traveller 5K, almost everyone will want continuous gravity, if only for health reasons.
While it's not mentioned, we can assume that the number of hardpoints is based on the actual size of the planetoid, which re-emphasizes the military value of it.
Not so clear on the actual number of hull points, though it is based on the actual size, and to compensate for the removal of the higher armour factor advantage, allows a great deal of more battering, by giving a twenty five percent bonus in hull points, and a fifty percent bonus for the buffered variant; what is less clear is if an interplanetary rock is actually a ship that also gives bonii at twenty five and hundred kay tonnes.