well if u buy a laptop HDD and put it in casing it will be priced same as external ones.
But if u r putting desktop HDD into casing it may cost u little less but it will still be around 3.5K again same as the price of branded external which are based on external power adaptor. Check this , the 3.5" (inch) ones are what which require external power adaptors.
So u see the benefit of directly buying a branded external HDD, also u wont ahve to deal with cheap casing of which warranty is not more than 6 months or something, whereas a branded package cosists of all, hence good to buy.
Brand Make Tech Specs Price
Seagate Free Agent 500 GB 3.5'' W - USB
2 Yrs Warranty 3575
Seagate Free Agent 500 GB 3.5'' W - USB
5 Yrs Warranty 3925
Seagate Free Agent 750 GB 3.5'' W - USB 4450
Seagate Free Agent 1 TB 3.5'' W - USB 5300
Seagate Free Agent 1.5 TB 3.5'' W - USB 8950
Western Digital My Book 640 GB 3.5'' Essential 4050
Western Digital My Book 1 TB GB 3.5'' Essential 5450
MAXTOR Basics 500 GB 3.5'' W - USB 3525
MAXTOR Basics 1 TB 3.5'' W - USB 5700
MAXTOR Basics 1.5 TB 3.5'' W - USB 9500