After moving in 4 Sims, it costs 5,000 to move another one in. Instead of spending 5,000, you can get two sims (I used male/female couples, I don't know if you can have a romantic relationship between the same sex) to move in with each other. You have to get their relationship status to partner, then you can move them in. Once you do this, go to the house of the other sim and sell all their rooms. (You will have to go under "construction" to sell the room). It will ask if you want to sell and add all the items to your inventory. If you don't want to add items to inventory, then cancel and sell items, then you can sell the room. After you do this, demolish the house. Then you can add a new sim to a house for cheaper.