have heard of people using a squirt of CO2 then wait a bit then gas them fully. I have been looking into this and have found that animals in general will not be able to remain conscious for oxygen levels below about 14% (Normally 20% in atmospheric conditions, correct me if wrong). I have think this is the cheapest humane method (short of WHACK on head). This is from reseach not personal experience though from what I've read and heard. I am yet to keep breeding rats (boa is the bomb for rat supply), I'd probably do the whack on the head method.