Ranks cost 25 ROBUX each. My group has ~30 ranks, 30 x 25 is 750. For me personally, it was more economically feasible to just pay the 100R$ fee to change the group name instead of making a new group with ranks (which wouldโve cost 850).
It fully depends on your group. If it has a few members and no ranks, making a new group would make sense.