Here's 500GB which seems to be reasonably priced:

http://cgi.ebay.com/Maxtor-500GB-7200RPM-16MB-USB-2-0-E...QQrdZ1QQcmdZViewItem

You just highlight the contents in each drive that you want to back up, click the "copy tab", which is usually located on the upper left hand side of your screen. You then get a pop up to where you want to copy the documents to, (which will be your new external hard drive) and then you're done.