something on which you keep having to spend a lot of money, especially when it may be a waste of money

We don't want the project to become a money pit. 我们不希望这个项目变成一个要不断投钱进去的无底洞。

Old houses can become money pits for their owners. 老房子可能成为要业主不断投钱进去的无底洞。