The cheapest and quite painless way is to use a gas. There are several to choose from such as hydrogen sulfide, hydrogen cyanide, or even Freon. The sulfides or cyanides interfere with oxygen transport and you go to sleep due to lack of oxygen. You do not feel air hunger as this is driven by CO2 levels in the blood. You are expelling CO2 and inhaling the sulfides that in effect kill due to lack of oxygen transport. This is similar to the mechanism as carbon monoxide but is its effect on hemoglobin. It will work also and is cheap to make.
Freon is actually the best. In high concentrations it is an anesthetic and will also cause arrhythmia of the heart. You go to sleep and die totally painless.
And the cheapest gas is nitrogen, which also does not cause air hunger.
N for the Win!