I have been wondering about what to do with my mice if I ever get my hands on any. I will of course only be euthanising pinkies and nothing bigger, so I have considered the freezer. But as far as a cheap CO2 chamber works, providing you aren't doing too many large creatures go.... this is what I am probably going to do:
Get a selaed tupperware/rubbermaid/whatever plastic box. Drill 1 tiny hole in the lid (to release the CO2 if the pressue becomes too much. At the other beside the edge drill a hole with some tubing going through. In the container under the tubing attatch a small contanier e.g. a camera film box to the edge so the mice can't knock it over or get into it. Fill it with cheap baking power, close the lid, pour cheap vinegar through the tube.
This reaction releases enough CO2 gas to do the trick if the container is reasonably small. And its very cheap.