I almost never get upgraded - even on expensive tickets. I think there are two factors - they are discounting first so more people are purchasing it and they are offering upsells for reasonable prices that people are buying.

Last week I was #15 on DEN-IAH on an M fare. I have no idea how this could happen. I am flying DEN-LAS next week on an expensive ticket and I didn't get upgraded because of the inexpensive upsells. Right now 1K is worthless.