If you've got a GBA and the GameCube Link Cable, you can access an island to the south of Animal Forest + that lets you make money much more quickly. Collect as many pieces of fruit as you can carry, hook up your GBA, turn it on, and then go to the pier at the beach (all the way in the south). A Kappa will be waiting here in his boat (he only shows up if your GBA is switched on) and take you to the island. Once you're on the island, drop all the fruit you have collected and go back to the boat. Tell the Kappa that you want to leave and that you want to transfer the island to your GBA. Your GBA will turn on and you will have see your island, complete with all the fruit you have dropped. Move your hand to the door of your island animal's house and knock. Your animal will come out. Now grab a piece of fruit and put it right in front of your animal. It will eat the fruit and turn happy. After a few pieces of fruit your animal will drop a money bag of random size for each fruit you feed to your little virtual pet. Repeat this process until all pieces of fruit are gone (you can also direct your pet to shake trees to get more fruit), then put your GBA to sleep. Do NOT switch it off.
Go back to the GameCube game. Once back in your town, leave the screen with the boat and come back. Talk to the Kappa again and the GCN game will be updated with the new data from the GBA island. Travel back to the island and collect all the money bags. Some bags contain as much as 30,000 coins! You should be able to pay off your house in no time now.