Closing up a colony to kill it is not foolproof, as not all woodenware is bee-tight at all joints. Survivors will drift.

A plastic bag is also not foolproof, bees quickly chew through plastic, as more than one beekeeper has learned when mistakenly using plastic window screen rather than metal screen as entrance closures when moving colonies.  Again, survivors will drift

CO2 is cheap, and easy to get from welding supply houses, or already on-hand.
Soapy water is far cheaper than CO2, but requires more labor/time with garden-hose-end pesticide sprayers (Ortho Dial-N-Spray), and may not be as quick and complete as CO2.
Sodium cyanide is very effective, used to fumigate comb to prevent wax moth infestation, but it's... cyanide.  You'd be crazy to use it.  Its illegal in the USA, moreso after the 2004 EnviroKem incident in North Dakota.

CO2 seems worth the extra expense, as one can refill fire extinguishers over and over from large tanks, and keep everything moving quickly.  Tragic work, to kill one's bees.  Might as well get it over with quickly.

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