RE:Portable millOf course demand is high that's a cool thing to do and has a Henry David Thoreau vibe to it. I'll buy one sooner than later.
Despite that, the gravy train of mall retail wages is too great a lure for me in this scenario you've proposed. A cursory google tells me experienced operators get maybe 100 bf an hour cutting spruce. So, 8 hours for 0.8 MFBM. There's an episodic elevated margin of the current prices less log costs less overhead... but you don’t get list prices. You take a big discount given it's (green) (rough) (unstamped) (ungraded). But sure, at just an $80/m3 log, 30% overhead, and $1000 episodic revenues, notionally is $520 per day. But half the time is driving and whatnot, so it's more like $270 per day.
The real fun is normalized pricing at say $500 to $700. Call it $600. My math is you're pulling down a sweet $101/day then. It's a hobby for rich guys, and a side business for value-added pieces (e.g. custom timbers). Cranking out commodity lumber is an economies of scale game.