I've seen the upgrade offer quite a bit on DCA/LGA and DCA/BOS both 319 routes (mixed with E175s). Although probably these were off times-middle of the week, middle of the day. Usually they're pretty cheap-$49 to $69.

I would agree that the offers are probably based on the historical level of booked seats F on that particular route. I for example never see them on MIA/DCA or MIA/DFW.

What I have noticed is that F fares have jumped quite a bit since summer. Typical now is $1.5K for MIA/DFW (on a 321). Given how full the F cabins seem to be even before the upgrade window opens I guess AA is getting these higher fares and when not aggressively selling upgrades.