I have done this on 4 different accounts to see what the issue was.
So the problem is - on certain accounts, you receive the prize 2 levels before. Some accounts you receive the prize 1 level after. Other accounts receive it on the level itself. Some 1 level before, some 2 levels before. So once you know the pattern from the beginning levels, you'd know whether it was worth it or not grinding all the way to 120 to get the prize.
But end of the day, the amulets and backgrounds are hard af to sell so I wouldn't bother anymore.
Edit:
@(you need an account to see links)
Is getting the prizes on different levels on different accounts a server-side thing? Some of my accounts got it on x8, some on x9, some on x0 but many of them got it on x1 which means you'll miss level 120's prize as they would award it on 121.