What's your experience level? How's your memory and attention to detail? Any curious kids at risk? Is the growroom sealed, or will the CO2 leak throughout the house?
CO2 isn't a toy, (it displaces the oxygen, and can suffocate mammals) nor is it necessary unless in a hot enviornment. And even then, I've personally never felt the need to spend the extra cash. (even though I live and grow in the SW desert)