I've used dryice many times, of course it depends on what you can get it for. Where I am, I pay about $14.00 for a 50lb block, which lasts about 4-5 days.Try Dry Ice Directory - Where to buy Dry Ice for pricing in your area.
Co2 is used only during the lights on period. Thats it.You may want to get a hose, and run it just above your plants, with holes drilled in it, pointing at your ladies, this makes a nice delivery system.