Oh, you want a cheap CO2 source?
OK sit in your grow room. . .
Voila, your exhaled breath increases the CO2 in that room.
See how easy that was?
If you need more CO2, then EXERCISE in your grow room.
Obviously, that's more work, though its still cheap.
A bit more seriously, I'd be **HIGHLY** leery of putting an open flame in my grow room. Also, if you are venting your room the way you probably ought to, then whatever little extra CO2 you'd be adding from something like a candle would probably just get blown right out via your exhaust.