Keep building more and more houses/merchants literally as soon as you get the money and experience level for them. Merchants don’t take long to pay HUGE sums of money early on in the game.
Earn additional XP and coins by completeing quests. To see the quests menu to start completing some yourself, tap on the icon to the left of the screen.
Keep in mind that you don’t have to buy land that is directly connected to land that you already own. Whatever the next place is that you can explore (such as a cave or a tower), you can buy land right next to it. Then you can expand to the land with the artifact on it, and explore it, without having to worry about owning the entire island.
Rubies do cost real money to acquire, and are not free. Only spend Rubies if you feel comfortable doing so, and if you understand that they can only be obtained by spending real money.