So total per month for each one in $/kWh is

$350.00-USD 1x 280x 720kh/sec 0.09$/kWh 24/7 x 30 days = $16.20-USD
$1200.00-USD 12x usb miners 720kh/sec 0.09$/kWh 24/7 x 30 days = $1.50-USD

You would save $14.70 per month on your electricity bill, however you spent 650.00 more for the USB miners. It would take you 44+ months to make up for the cost.
So is it worth it? Not with AMD pumping out 20mn chips soon.

Also quick resell value of the 280x, around 300.  USB chips that only do two things? Not worried about ASIC for scrypt at all.