Zero,
Why would you consider the dive on air, if you would do the Deco on EAN? You are already investing in the EAN gas, so there is no additional expense. I do alot of tri-mix, with deco on 50% and 100%, and I can not even recall the last time I used air. I FIRMLY believe in the use of a gas that allows me an additional measure of saftey. EAN is not the " be all and end all" of magic gas, but it does afford me better options when diving.