The only 1TB HDD that I've seen for under $50 was last month when TigerDirect had a Seagate model for about $40 after rebate.
If you're going to use the new HDD to store your operating system, ewiz has the Samsung F3 1TB for about $60 (use code SUNSHINE15) with free shipping.
If you're just looking for a storage drive, Newegg has the Samsung F2 1TB drive for $55 (use the code on the page before it expires on July 11) with free shipping.