I contacted Herman Jansen and they told me the Mansion House is only for the Asian market.
Indeed, it is only for the local market which is obvious.

Who the heck would want to drink Mansion House outside here?

What I meant was that Mansion House was produced in Semarang by Industri Semak (they also produce other bad brands such as Mc Donald Whisky and other poisons) under the license/supervisation of Herman Jansen which is a Dutch brand.
No problem to get it here, price is 280K
Rp 275K/280K for the Captain Rum is about the right pricing in JKT or nearby. Typically wine shops/retailers add a 20% mark up. The farest you live from JKT, the more expensive it will be at the retailing level.
It is never a problem when you want 1 or 2 carton. The problem starts when you want to get a sizeable bulk. Diageo Indonesia simply can not sustain the demand for the Captain Morgan white rum, especially a peak time demand.
When traders like us request 25 cartons of Captain Morgan white rum, we end up with half of that, while for the Spiced Gold the problem doesn't occur. Therefore we don't really care if Manta white rum costs an extra Rp 20K or even 50K.
Booze in Indonesia is a seller's market before all and If I order 25 cartons of white rum it is because my market is asking for it. If I end up with only half of what I need, which happens to me pretty much every month, I have no other choice than ordering Manta white rum which is the "second best choice" when considering price and quality, and this even though it is more pricey for a very similar quality.
The situation for the Spiced gold is totally different. Production easily meets the demand. So if Manta continue to be overpriced compared to Captain Morgan, they will shortly realize that they have to reconsider their pricing.
Manta Dark Rum has no real competitor* (Captain Morgan do not produce Dark Rum here and Mansion House Dark/Jamaica Rum is an adventure) so they can get away with an heavy pricing (it is around Rp 25K more expensive than both the Spiced Gold and the White Rum when bought at source and more than 50% more expensive than the MH thing).
*I am talking about legal/not smuggled booze