if you want to save money CO2 cylinders and gas cost some serious coin and there are cheaper options.
Ive made my own Co2 chamber for about $5. plus Vinegar and Bicarb.
Works every time and very easy to make and use.
heres some pics of mine and a link on how to make.
In the link use the euthanasia machine method as i did rather then the bag job. it would have better co2 kill rate.
less human error.
http://www.alysion.org/euthanasia/ 
