[2020] Diamond cost reduction?

Gaming and Virtual goods are a bit different. This is why all items in the game have an intrinsic value, this allows us to prove the worth of anything in the game. This then allows us to make various offers on items, like the one you cite.  

It is a complicated algorithm that takes a lot of things into account. It's effect also has some value if you calculate its benefits. It is quite complicated, and we can't get there, but they also have an intrinsic base value.  

@Pauly7 You are correct, there are laws in place to ensure that consumers are protected against such practices. All companies are well within their rights to lower and increase costs in order to remain in a competitive market however in the event of Sale Items the previous price listed must be clear to see and have remained at that price for a "meaningful length of time" in order to meet lawful requirements. This of course stops price hiking prior to a Sale Event.  

Pauly7 said: So is it therefore demonstrable how a Dwarven Armourer has an intrinsic value of 1,000 diamonds?

Yes and this is the same for every building we have. The calculations are done from before any item is even introduced to the game to make sure that everything is taken into consideration. A term we call balancing as this takes into consideration the power of the building, how common or rare it will be, how hard will it be for a player to get, as well as its footprint and of course the maths formula that each aspect of this creates.
As a company we have to be able to use a consistent formula all across the game ''balancing'' is exactly this.
Therefore when any offer is made like today, this is based on a set of known and fair pricing that adheres to legislation.  

We would be able to defend this @Pauly7. As an international company we also have a legal team to make sure that we are well within the law and operate in a consistent and fair manner.