If my sub is rated

2000 watts Dynamic
1000 watts RMS Power

how much am I consuming in real life? Say I am using half of max volume.

Thanks,

This is impossible to know without measuring. Any guess would be just that.

Realistically, for normal output, not too much.

Half power = -3dB

Half output = -10dB(or 1/8th power roughly 2³)

So assuming you are actually getting a measured to the human ear 1/2 max output, that already puts you around 125W RMS.

Average amplifier output could be somewhere as low as 20W, probably would be drawing around .25A accounting for efficiency at about 80%.

But as noted above, it's completely made up without actually measuring :)