The cost of an external hard drive generally depends on how many gigabytes you are actually in need of. The one-touch western digital hard drives are exquisitely designed and priced based on their varying capacities. For instance, to replace an external hard drive in a computer, it costs about $200 for a 500 GB in a Macbook. Some of the latest types can contain over 300,000 MB photos and more than 250,000 MP3s in them. The average cost of an external hard drive is about $100.