In the Light Novel Sherry talks them down to 20 silver per mirror... And it's not really discussed anywhere, but the 1 silver coin entrance fee is probably used to make the palmasque mirror, which requires silver to reflect. Keep in mind that palmasque doesn't have a nearby labyrinth, so there is no Silver Cyclops that would drop a silver coin.
The time difference to the mid point Zabir is 2 hours, so all the way to Palmasque would be 4 hours. Since there is a 24 hour day, their planet is likely equal in size to ours. So a 4 hour time difference would be equivalent to 1/6 of the way around the world, or 1300 miles.