So I can shed a little bit of light on why the price bounces around so quickly. The wholesale cost of the drives is $159.00. If they're at $159.99, they're using them as essentially a loss leader and making a grand total of $1.00 of profit off of the drive (likely actually losing money because of associated costs with transport, storage, labor, etc.). They put them on sale frequently because they have access to a rather large supply of them, and they're exclusively provided to Best Buy (the whole easystore line is a BBY exclusive they negotiated with WD) at low wholesale cost because BBY at this point stocks almost exclusively WD drives, at least in stores.
Also someone asked above if there is any way to tell from the outside packaging whether the drive inside is a WD80EFZX with 128mb cache, or a WD80EFAX with 256mb cache. Short answer: there is, and I've only seen one report of it being inaccurate. If you flip the box over, on the underside with the barcodes it should say Manufactured in Thailand or Manufactured in China. The Thailand drives are the 256mb variant, Chinese variant is the 128mb variant.
Also, if you purchase a drive and it's within the 15 day return period, you can still price match the new sale price so long as it's not an hourly sale (yes that's a thing) or during Thanksgiving weekend (Thurs-Mon). In-store you'd just take in your receipt and speak to customer service, online is a little bit trickier but you can do so by contacting the corporate 1-888-BEST-BUY phone number.
