Most UA domestic upgrades are calculated based on fare differential and are actually relatively easy to predict. On the premium and long-haul routes (including Hawaii) you're more likely to see upgrades priced independent of the fare.

To your point, AA utilizes fixed price upgrades on many routes, and I frequently take advantage of this to get an F seat for considerably less than an F ticket would cost. It's really a way of discounting F without having to mess with the fares themselves. It can be debated whether AA is leaving money on the table by doing this (in general, their RM is much less sophisticated than UA's, so the answer is probably "yes").