Well you could put in pull-down resistors to tie them low. That shouldn't consume much, if anything. Pull-ups will add to the current drawn (as you would expect) according to my measurements here:http://www.gammon.com.au/forum/?id=11497
If the input buffer is enabled and the input signal is left floating or have an analog signal level close to VCC/2, the input buffer will use excessive power.
An analog signal level close to VCC/2 on an input pin can cause significant current even in active mode.
According to my measurements on the page quoted above, the difference is about 0.9 uA.I presume where the current is "going" is making the input MOSFETs "work harder" by amplifying noise.See page 43 of the datasheet:
PORTB = PORTC = PORTD = 0xFF; //enable all pullups
an analog signal level close to VCC / 2