A couple of things about CO2. I have used both extensively...so.
1) CO2 is NOT inert. It is an active or oxidizing gas. It does introduce impurities into the weld as it is superheated. The top of the weld isn't in the condition it is because it isn't cleaning...it is in the condition it is because it is oxidized by the C02. CO2 does better than nothing for sure, but it isn't ideal for a quality weld
2) Voltage required for the arc to bridge the gap using C02 is higher. This means you are naturally going to run hotter, and have a more violent AND unstable arc situation.
3) Clean up expense and time, if you are into that sort of thing (making a weld look clean and nice), can make up for the clean, pure look of the weld, because much higher rate of spatter also sticks harder when CO2 is used.
4) Regulators, even CO2 ones, will freeze. They make "heaters" for the regulators you can use.
5) Gas flow rate requirement for CO2 seems to be higher, so consumpution is higher.

As far as the price difference, I have kept up with my labor for both, and the specific blend I use, and for me it pays to use a higher amount of Argon, even though it is more expensive. I personally use an 18% CO2 content.