To answer this topic, we should back to the supply-demand and inflation theory that usually happens in economics real life which is same with in game.
Supply and demand is perhaps one of the most fundamental concepts of economics and it is the backbone of a market economy. Demand refers to how much (quantity) of a product (Membership Medallion) is desired by buyers. The quantity demanded is the amount of a product people are willing to buy at a certain price; the relationship between price and quantity demanded is known as the demand relationship. Supply represents how much the market can offer. The quantity supplied refers to the amount of a certain good producers (in this context we called it Donator) are willing to supply (MS) when receiving a certain price. The correlation between price and how much of a Membership Medallion is supplied to the market is known as the supply relationship. Price, therefore, is a reflection of supply and demand.
Imagine that there's 100 persons who need Membership Medallion, while we just have like 20 donators in this game which donated to the server. In before the prices around 300k, but because the buyers number increase upon a time, the price will subsequently rise because, according to the demand relationship, as demand increases, so does the price.
In addition, inflation happens in every game, as time passes more money gets into circulation and as more money enters that circulation prices start to shift as some people either don't mind spending more, or will expect more.
This is an interesting question, why just MS? why mounts and other items are steady?
Just wanna try to answer this question, in my opinion its because MS is a primary needs in this game while mounts are secondary or even tertiary needs for some people. E.g. we need ms to hunt membership pokemon, bonus exp and double money gain, etc, while we use mounts just for style and we can substitute mount with bicycle in game.
To sum up, this happens in every server and this is normal because of inflation and supply demand theory. Btw sorry for the WoT. xD