Septim is the currency in the game. It is also the name of the royal family. The whole game you are trying to return the amulet to Martin Septim after his father dies. When you finally do during Dagons invasion, Martin Septim becomes a dragon and sends Dagon back to oblivion. Then he turns to stone and dies.

The cost of the main quest is one septim as in martin septim, not the money septim.