Average $15-$25 with about 100mb of storage space
There are many different types of "disk drives" with a variety of prices. Please be more specific as to what type of drive you are interested in purchasing.
Floppy disks are seldom sold individually. Depending on the bulk number you purchase, the price per floppy is generally $0.75 to $1.00 USD.
$25
the hard disk can be thousands of dollars. The maxium is $749.