You can’t really estimate this because we don’t know how much you’re storing (also that we can’t guarantee how much memory or CPU or storage each postgres instance would actually use in any given environment). I’d say just deploy your instance and see how much $ worth of resources you’re spending per month.

if there is not much load, start with 2 cpus. If you know the avg queries per second and avg runtime. You can calculate the cores as 1.25 x queries per second x avg runtime.
A good initial sweet spot would be 4 cps for moderate load.

Ram is important. Keep your ram at least 25% of your database size.

For storage it would be
Backup Storage = 4 x (Db Size x 0.4)
This is for 4 backups that will be stored in the month and assuming you are using compression to store the backups. Change number of backups according to your needs.

If you are going to store your backups on a storage bucket, then at that time your egress would spike. So for a 100GB backup that would be 100 GB egress x 4 times in a month.

Ingress is normally lesser that Egress on an avg in the ratio of 1:5.
It is difficult to estimate the egres/ingres

for a 100 gb db with your backups I would assume 1tb of Ingress and egress in total.