cheapest will be...buy an ordinary HDD and a cover for portability( which have USB to bus coonection ) ...that will be cheapest . hard disk's only feature is storing data . I dont think it have some other features , well depending on the disks u may get SATA , pata, 5400rpm 7200 rpm or 15k rpm .
u have 2 options for HDD's :
3.5" HDD ( it requires an external power supply ) - the usual desktop HDD
2.5" HDD - gets power from the USB port - laptop HDD
not too sure about the prices ,but i think u can get 40GB 2.5" well within 3k