Seagate Barracuda 500GB 7200RPM Drive for $39.99 shipped after applying coupon code ZZF100915 in cart and submitting a $15 rebate.  Here is a screenshot before the rebate for your reference.  Note that it is in OEM packaging, which means no pretty retail box and marketing materials.  It may be interesting to know this is the newer 7200.12 model, which utilizes a 500GB/platter design, the highest density currently available for reduced power consumption and better performance.  Most places have it for about $54.