As anonimous said, you gotta get top 50k to get the rewards. But the rank depends only on how much time you played for. If you don't pay any money then you can only play for like ten minutes at best every five hours. So basically it depends on how much other players are willing to pay. You can use jewels you get for free to play more, but they're pretty limited.
There's a high score rank which in theory is skill based. But you need 5 SSRs to have a chance to get high ranks, which needs either luck or money, and even if you're good enough to hit perfect on every note the high score is based on how lucky you were with the RNG based abilities activating.
Basically what I'm saying is the ranks are a sham and mobage devs need to be sent to the gulag