I have a short flight in July: San Diego to Phoenix. The upgrade on the app was $450 (a bit more than twice the cost of the original ticket, and thus more than just buying an F seat.)
The actual first class fare was something like $745 one way (good grief!) That flight is less than an hour.
$75? $100? Sure. $450? I think the main cabin will be fine.

If I understand the American TOD upgrade system correctly, they tend to offer these within a few days of departure, is that right?