there is another anser to this co2 dilema!..
a simple one, but effective.
increase the airflow of fresh air into the grow chamber, the amount of co2 in the atmosphere is sufficient for a very good grow! also i believe if you run your system at night, there is a naturally occouring higher level of co2 caused bty all the outside plants exhaling and producing c02, s opposed to oxygen! the nearer to the country side you are ,[ or a well planted patio or garden] the better but the principle is the same draw the freshly co2 enriched air of the patio and double the airflow to the room..
there was a system used where to rooms where ran at opposite times and the co2 enriched air was punped to the flowering room during lit hrs and reversed at night!Remember that plants create oxygen whilst in photosynthesis, and co2 whilst not!
a kilo of bicarbonate of soda seems to be an expensive alternative to co2 injection in the long run!
so i'd save my money and buy a co2 rig!
happy growing.

welcome to the world of endless possibilities,,,,hydroponics!