I run a Trumpf TruLaser 3030. It's a fiber laser, and is a 5x10 machine. Before this one we had a Mitsubishi CO2 laser that was also 5x10. I have no experience with the small machines so I'm no help there. But I know with Nitrogen on steel (we use it on 9ga and under for travel speed) we run around 14 bar. We don't cut much stainless so I don't know the pressure for it off hand but the machine feeds at 27 bar, and stainless does use more pressure than steel does.
We normally use O2 on 3/16 and up unless customers want a laser scale free edge then we will use N2. For thin material it is more cost effective to use N2 because we can cut so much faster with it.
We also can use compressed air. We don't use it very often but if you have the compressor requirements and driers already it is the cheapest gas to run for steel, and you can run a decent speed with it too.
Hope this helps in some way, sorry I don't know any numbers on actual amounts used, I just run the machine..