Here's a quick PSA for anyone who has been looking to pick up Apple's iPad mini: The 64GB version of the diminutive tablet is back on sale for $400 at Amazon and Best Buy, matching the lowest price we've tracked. Though we've seen the 8.3-inch slate hit this price severaltimes in the past year — including for most of the recent holiday season — this discount is still about $30 off the device's average street price in recent months and $100 less than what you'd pay from Apple directly....