I've had instances where I call a month out and an upgrade to biz is $1k. Day of departure (T-24) still $1k and then at the check in counter I've gotten CPU'd (but that was due to irrops, late flight day before had a bunch of people change flights overselling Y and 15 open seats in biz) and I've also then gotten offers for $2k at check in - so it goes both ways.