When you go in City Hall, do not bother building the "Rat Cave". Tristan Legend will build it at no charge with additional progress in the game; however if you do build it, it will cost 3,000 Simoleons.

Use the following trick to get the Rat Cave for free. Do not buy the Rat Cave. Tristan Legend will call you and say that he has something to show you. You will then go to the basement. He then shows you that he built the Rat Cave.