$250 will set you up with a tank and regulator, and a simple timer to control release.
It's the safest and most efficient way to go about adding CO2. My feeling on CO2 is this:
Get your other variables squared away before even considering it.
When it is time to supplement CO2, don't bother doing it half-assed. The yeast method works tolerably well but stinks to high heavens. Candles are not practical at all- dangerous and seriously the amount of carbon in the paraffin isn't really all that awesome, I mean, it is super lots of carbon by % of mass, but not much mass, you know? plus you are adding heat.