I think it is less about cost saving and more about getting to the root of the problem. According to Elon's statements, Lidar is a crutch and since the AI will eventually have to be good enough to remove the crutch, remove it an the get go and save learning how to use the crutch. This is similar to his reasoning about EVs. Since we will eventually have to have petroleum free cars, Lets remove them as early as possible.