You can install an airdam in a few minutes for a few bucks that will make a positive difference on your fuel economy.
You can install a belly pan in a few hours for a lot more bucks that will make a bigger difference.
I read from someone else that you could get about 80% of a full belly pan's benefit with a well-executed air dam, for a fraction of the pan's cost. Seeing how I have neither, I have no actual experience to speak from, I'm only repeating the wisdom of others.
One thing I have wondered is whether there is a benefit to be had by doing both. In the case of a fully panned car, I strongly doubt you would need as much dam.
Take it easy