The pricing isn't always predictable, and often gets more expensive as you get closer to departure. As an example, this week I had the chance to buy an F upgrade on UA for about $310 shortly after it was booked (about 6 days out). A few days later it was up to $700, and by the day of the flight it was $2300. UA does similar, though less extreme, pricing with E+ - it generally seems to go up by $10-30 between when the ticket is purchased and the day of the flight.

Pricing for premium and F upgrades is probably more likely to drop if load factors are low, but lately I've mostly seen them going up rather than down as you get close to flight. In principle, AS sells F upgrades cheaply at the gate, but I haven't seen that in a long time, since load factors seem to be high and there are enough elites to take all the U space with free upgrades.