The Torah tells us that when you appoint a king, he should not accumulate horses or wives. In 17:17 it says, "וכסף וזהב לא ירבה לו מאד" - silver and gold he should not accumulate too much. By the others it just says not too accumulate to many but by money it adds the word מאד - too much.
What is the difference between them that money gets a stronger warning?
It costs a lot of money, and requires a lot of money, to run a country and government. A king does not require a lot of wives. He does not even require so many horses (some for his army, but not so much). But he does need a lot of money.
This is alluded to in Rashi who brings a drasha that he should not have an abundance of money - for himself, but for "achsanya" it is ok. "Achsanya" is defined as money needed to pay for army and servants and the like - i.e. basic functioning of government and the necessary services for the country.
So it is ok for him to accumulate the great sums of money he will need for the functioning of the government, just not for personal needs.
So a king needs a lot of money for the functioning of his government, but he should not look to accumulate too much (מאד). It will become a burden on the nation (via taxes). He should not be looking to increase his personal wealth by being king.