Depending on what you're looking at upgrading from/to prices.can go up, down, or stay the same.

For example Economy+ seating (not truly an upgrade but many people think of it that way) is generally a fixed price for that seat.

Some economy to Premium Plus (UAs premium economy product) can either be a fixed diffentia or can float based on cabin sales (close to departure goes up if heavily booked or goes down if lightly booked). Polaris (the business cabin) typically also floats relative to bookings but that isn't necessarily true for all routes.

Moral of the story is generally if there's an offer at the price you're willing to pay grab it. Like shopping at Costco there's no guarantee you'll ever see it (or something better) again.
​​​​​

​