If you are installing hardwood flooring over concrete, you must add a floor vapor barrier between the concrete and the wood. You will often find that it is a requirement of the warranty coverage for the flooring. If you install the hardwood directly on the concrete, moisture from the soil will evaporate through the concrete and penetrate the wood, causing it to warp and allowing mold to grow. It is worth it, in the long run, to follow the proper instructions and invest in a floor vapor barrier.
White vapor retarded paint usually costs between $20 and $40 per gallon. One gallon will give you nearly 2 coats of protection. Using 8-mm plastic sheeting in a 12x12-foot room, assume that the sheeting will cost between $.25 and $2.50 per square foot to estimate that the project will cost anywhere between $36 and $215.