Newegg has the Western Digital Caviar 640GB 7200 RPM 16MB Cache SATA II OEM Hard Drive for $85 - $5 (with code EMCAJCGBK) = $80 with free shipping

List Price: $85
Mail in rebate /Discount: - $5
Sales price: $80.00 + Free shipping
Price After Rebate: $N/A

Technical Details:
Model
Brand Western Digital
Series Caviar SE16
Model WD6400AAKS
Performance
Interface SATA 3.0Gb/s
Capacity 640GB
RPM 7200 RPM
Cache 16MB
Physical Spec
Form Factor 3.5"
Manufacturer Warranty
Parts 3 years limited
Labor 3 years limited

Link to the buy: Newegg.com
Rebate form: Code EMCAJCGBK