Is it really energy economy to use movement sensor for turn lights on?
Can't find that one. Another company's model that gives numbers (after looking more - MANY don't, which seems sketchy) claims 10mA @ 24v, or 0.01A x24V = 0.24W - less than 15, yes; Quite a bit more than 0.002, though. Whether it's actually worth buying (since you mention that it's expensive) would depend on how much you pay for power and how many hours it would save you 14.75W (or so) - it might take many years to pay for itself, unless your power is also very expensive. 

This link has a rather more pessimistic assumption about the sensor at ~3W or so.

And this link has some specific numbers from another brand, ranging from 0.06W to 1.04W (1.04W is a dual-sensor/ ultra-sonic/PIR - highest is 0.18W for PIR only)