Curly and Moe are considering living alone or being roommates and splitting the rent for the next twelve months. If they live alone, they each rent a one bedroom, one bath apartment for $500 per month, while if they are roommates, they can split a two bedroom, one bath apartment for $800 per month. The one difficulty they have is that Moe snores very loudly. Curly estimates the cost of poor sleep due to Moe's snoring at $150 per month. Moe could obtain a snore-eliminating device for $50 per month. The least costly solution to the externality present in this situation is for: