USB would be "cheapest", if WIFI and Bluetooth radios are turned off, and the phone is already charged and the display stays off.
However, plugging in an iPhone in particular causes the display to go on bright and stay on, and that creates a significant current draw. Other phones may behave differently. [NB: Actually, I discovered that this was because the phone was simply set to not turn off the display.]
Bluetooth would be the second choice, especially if you can turn on Bluetooth without WIFI (which is a bit of a trick, since they share much of the same hardware).
If you need WIFI on for other reasons then it probably doesn't matter much between WIFI and Bluetooth.
Added: As I stated above, I discovered that the iPhone display problem was simply a settings problem. Fixed that and the display turns off after the timeout. So the USB tether should always be the lowest current option, if you can turn off the radios.