Hello,
in my personal opinion as a start for a new server there shouldn't be too many Donator ranks, I would add 5 or 6 ranks to make it easier and more simple to make, here is my suggested list related to the space theme:
Earth - Default rank
Mercury - 5$
Venus - 10$
Mars - 20$
Jupiter - 30$
Saturn - 40$
Sun - 50$
Other planets or names related to space could be added in the future for additional Donator ranks or from special events. Hope I've been helpful for you ;)!