It's all about balance. The amount of light you have on your tank will determine your need for co2. The plants will react to the higher light levels by growing faster. This means they are sucking up nutrients like crazy. When you increase light you also have to increase nutrient levels. This is done through adding fertilizer to the water column and tabs in the substrate, or a nutrient rich substrate. The limiting factor then becomes co2. So adding co2 will balance the system and let the plants use as much of the nutrients and light as possible. When the system is out of balance algae is usually the end result. All that being said, temporarily increasing the co2 till you are happy with the results would only allow you to achieve those results temporarily.
As has been said the initial cost of setting up co2 can be expensive:
Regulator, solenoid, bubble counter, co2 tubing, diffuser, check valve, co2 tank.
After that, my local welding shop will fill me up for 15 bucks. I will have to do that like once or twice a year in a small tank such as 10gallon. I use a 5lb tank. You can get larger tanks that will last longer.
