What does the rules say about the Mercenary adventure card.

Is it only necessary to pay the mercenary 1g once you pick him up and for each turn you decide to use his stregth?

Does this mean that I could pay him just 1g when picking him up and if I never used him in the game I would not have to pay him anymore?

That is correct, after the initial payment of 1 gold to aquire the Mercenary, the only other times you would pay a gold would be each time you use the Mercenarys Strength to add to your own.