Some of my reports may have been lost all the other posts but for travel in May, I've seen the following buy-up offers since first purchasing my tickets (i.e., buy-up offer hasn't changed since Jan 30th when I bought my tickets):
IAH-BOS, $208
BOS-DEN, $216
On Nov 25 2018, I paid $153.69 to upgrade my next-day HNL-SFO flight from Q class (waitlisted for CPU) to Z class (confirmed domestic F). At the time I found the offer and confirmed the purchase with CS, even she remarked "that's a great fare!"
I was getting upgrade offers for my DEN-FRA flight in June ranging from a couple thousand to about 8000 until I applied a GPU. Haven't received a cash buy-up offer since a couple days after applying a GPU (which still hasn't cleared).