Literally 30 seconds of searching brought up a IFF transponder that would add 100 g (3.53 oz). Why isn't a state owned drone not equipped as such? (XPC-TR Mode C Transponder)
As to transponder power usage, it would be very negligible compared to the power draw of the ESC/motors.
Even if it were equipped with the transponder it is not a given that F-15, as flown, would be (A) interrogating it, (B) show the pilots the replies, (C) show the pilot the replies early enough to make the difference. Pilot flying F-15 at 500AGL is likely training for other things than avoiding small drones. I think at those altitudes and speeds he is given the clearance to exercise (500+kts; not the regular restriction of under 250kts below 10000ft) in a well defined region and assumes that it is clear.