Alex said:

Why did I have to buy silver coins for this?

Because Plarium wants your money. All of it. 

There really isn't anything deep here. Silver coins can be used only for this building and you buy them for rubies. So, "silver coins" = "rubies", but they kinda obscure the amount ("3 silver coins" sounds better than "150 rubies"... and the last level is "1000 silver coins", which sounds OK(ish), unlike "50k rubies").

Why is it not shown in the "cost"? Likely because the coins were hastily added, like most other "features", and nobody thought to add them to the "cost" too.