The cost of plutonium is not 8000 USD per gram when manufactured. Generally, these are manufactured in reactors that geenrate electricity too. So, the plutonium can be considered as a by product. The cost of reactor too, is merely a notional cost as the actual cost of R&D etc are very old and are strategic in nature, not business type. So, the cost of plutonium is not really high and is mostly recovered by distributing the electricity generated. Also, the nuclear warheads are continually upgraded and materials changed/recycled to ensure that they are operational and hasn't deteriorated. The Uranium/plutonium are are also recycled. Generally, most of the cost is fixed costs under the salary component and not capital costs.
The cost of Uranium is $50000 per ton. It has 7.1 kg of U235. So, per kg U235 cost will be $7000. 1 ton of natural Uranium can give 5kg of plutonium in PHWR reactor if production run is used. So, 1 bomb per ton of Uranium can be made with material cost of $50000. Since the other material costs are also not too high and the remaining cost is labour which is fixed cost of salary, the cost of a fission bomb is not really high.
Trident missile can use warheads of either W88 or W76, for example. Similarly, the size of the warhead can be varied on other missiles too. The gap between the cone can always be filled with some ceramic or light weight fillers.
Yes, warhead weight cannot be changed but an entirely different design of warhead can be loaded on the missile, such as the example of Trident mentioned, two different types of warheads, either of which can be loaded on Trident.
The cost of plutonium I mentioned was retail, but if made by country themselves, even if making costs is lower , the time frame needed of extract a kilo of plutonium from uranium fuel rods is too long a wait.
Do look into the physics of it, how many atoms of uranium actually convert to plutonium per million and how long it takes.
Since missile mountable nuclear warheads have to be compact there is significant amount of plutonium used, as pure uranium wahead will be too heavy for same yield .
Keeping In mind all above, countries don't design a separate warhead type for each missile. Only a few box standard types are designed and missiles made accordingly.
From Pakistan's point of view. Ghauri series uses the initially designed 700 kg warhead.
Shaheen series has two variants.
Ghaznavi has a separate design and was a test vehicle for many years .
In total we have 3 or 4 warhead designs shared between.
Two types of Ghauri.
Abdali.
Ghaznavi.
Shaheen, Shaheen-1 , Shaheen-1A of which there are two variants.
Shaheen 2.
Shaheen-3.
Ababeel.
That's 11 types of strategic ballistic missiles Sharing just three warhead designs.
Then a special variant for Nasr
And just one nuclear warhead design shared between all Pakistan's cruise missiles.
And we have three types of Babur and two types of Raad ALCM.
That's one warhead design for 5 types of cruise missiles.
The grand total would be .
17 types of nuclear capable missiles sharing 6 designs of nuclear warheads.
.