A full automatic system won't leave much change from $500 dollars. Buy full automatic I mean regulator equipped with solenoid and bubble counter, and pH controller controlling the level of Co2 in the tank.
The model the pet store they want you to buy, less expensive to start up, but replacing the cartridge regularly can add up fairly quickly, while a full auto Co2 system only needs the Co2 canister refilled. Depending on the size of the canister and the planted aquarium, that could mean years before the canister needs to be refilled.
Apples and oranges actually. Full auto system is the most efficient and safe way to add proper amounts of Co2 to your tank. The regulator with solenoid and bubble counter, and the pH controller are the most expensive parts of full auto, but once you have them, they can last for decades. The cheaper option means you'll be a slave to the replacements as long as you use Co2, which can really add up over time. You'd need a drop checker to make sure the cheap unit doesn't add too much or too little Co2.
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