on the practical side, is the wire your using compatible with 100% c02? and will it produce acceptable welds at the same deposition rate with similar technique? I.E is this an easy transition?
at one job I did a large time study and cost evaluation of all wire feed welding for a shop. Off the top of my head gas usage accounts for less than about 2% of overall welding costs. So little in fact that 100% of the time a more expensive gas was choosen if it showed: a lower defect rate, a mild increase in deposition rate, or had a larger parameter range or was just plain easier to use. It's very easy to get focused on the consumables cost of welding which is overall about 5-30% of total welding cost, and not realize that a relatively small savings such as changing shielding gas, creates a bigger expense such as increased defect rate.
The old phrase " Penny wise, dollar foolish" comes to mind
Then again my knowledge of FCAW details is weak; if I remember correctly, 100% CO2 gases generally give good penetration and bead characteristics, albeit with more spatter.
just the ramblings of a madman....