How much does a hard disk cost?

You can get them pretty cheap nowadays. It ultimately depends on two factors. 1--Whether it is internal or external (Usually internal drives are cheaper than external ones)

2--Capacity in GB (the more gigabytes, the more money)

I just bought a Western Digital Caviar Blue 320 GB internal hard drive for $28 including taxes as well as a Hitachi 500 GB external hard drive for $54 including taxes just to give you an idea. A good place to look is Newegg online or you can try any decent computer, electronics, or office store locally wherever you live like maybe Fry's Electronics, Office Depot or Staples.