once you have to start thinking about low/zero electricity costs for a USB-based mining device in order to make it mine profitably, it's a fairly safe thing to say that you shouldn't be touching the whole thing in the first place. A typical computer has 4-8 usb ports, USB providing 5 volts and about 0.9-1.2 Ampere, which in turn means that we are talking about 5-7 watts per device, the BFL/Jalapeno is said to mine at 5Ghash/30 watts, i.e. 1 Ghash/6 watts. So do your own maths.