An 80 of 75/25 is around $65 where I live. A 20# (IIRC) of CO2 is $21.
I've been welding hundreds of hours with a MIG, maybe into the thousands. I do notice that the mix leaves a prettier bead. But, is it worth the extra money? Most all I do is thin tube. If it's thick, I pull out a rod, and I am getting my first roll of innershield to try on items where pretty don't matter. (I always liked the look of a 7018 on anything thick.)
When I had an ornamental metal business, I used CO2 exclusively, and it was fine. I bought one tank (a big one) of mixed, and never bought another, as the only thing I noticed was a narrower prettier bead. And a lot more money.
Please tell me your opinions. Where's mixed better? Where's CO2 better? Is there enough difference to justify three times the cost? Only downside I see is buying a cheap CO2 reg.
Steve