Depends on size of room but with tanks, lines, regulators, C02...and so on, I have to believe that unless you have a pretty damn big room that bags would be cheaper. I have a friend who has a 4k room of decent size and he has 4-6 hanging in there, total cost on those bags were $120 bucks, that's super cheep for the result you get from C02.
A caution though, Your room has to be sealed pretty well for these to work, that's why it works in my tent works so well. If you had a basement with vents, cracks and and windows and stuff it might escape before it can become effective. So far for what ive read and those I talk to these work best for tent, closet grows or as I said above really well sealed and controlled rooms.
