If you want to spend a minimal amount, every two weeks when there's a gacha rotation, you can spend $1 and $3 for two separate single rolls on the lim 1 and premium tabs, so $8 for a total of four rolls, so you can save your FGs for the 11-roll or for other things (expansions, refills).

Every month or so, there's also a guaranteed ticket gacha for spending 50 FGs, so you may want to save for that as well.