The prices in this shop are quite astronomical when you first discover this location. Don't be discouraged however, the weapons here become affordable as you reach later levels in the game.
Do you see that round teleport tile? It becomes accessible later in the game so you don't have to walk across the whole island - simply teleport from one of the houses in the village!
You will encounter these giant spiders in the caves near Terra. They drop 250 gold each. The same spiders outside the caves drop 120 gold. Farming these is very easy just be sure to stock up on some antidote.Poison
Keep a good supply of Antidote with you, as you get further into the game there are more and more enemies that can poison you. You can also learn the Antidote spell, which is good to have on your characters that are not the primary magic users.