how much does sata cables cost? and is replacing a HD that simple, meaning a 5 year old can do it?
Yep, it's that easy.
A SATA Cable will cost no more than 5 bucks. One is about a dollar at my local computer store. Also if you do plan on replacing the HDD don't forget attach the cable from the power supply to it . Pretty much whatever wires you removed from the old one, put them on the new one.