Just to be sure I've understood the rules correctly about ships...
At the beginning of the Movement phase:
-I pay 1 treasury/token for every ship already on the board (or the ship is scrapped)
During the Movement phase:
-I pay 2 treasury/tokens to build a new ship
At the end of the Movement phase:
-I pay 1 treasury/token for every ship that I DIDN'T move this turn (or the ship is scrapped)
It's that right? So to mantain a ship that I don't want to move, I've to pay 2 treasury/tokens (one at the beginning and one at the end of the movement phase)?
BUT, I could always move the ship back and forth to avoid this upkeep? Or the rule means that it must end the movement phase not on the starting point to avoid the final upkeep?
No. You pay 1 to maintain an existing ship when you move it, or at the end of movement for any ships you didn't move. So each ship only costs 1 to keep for the whole turn. In advanced civ this was a separate step before movement, but including it as part of movement makes the game flow smoother.