It varies greatly depending on your setup.

Lemmy.world's monthly expenses are around €1,200. Though that includes hosting more services that just Lemmy.
https://blog.mastodon.world/

For comparison, it looks like £30 per month for lemmy.zip:
https://lemmy.zip/post/7766703

And lemmy.ca is around CAD$30 per month:
https://sh.itjust.works/post/39134

At one point Reddthat.com was considering moving to a bigger server that would cost A$150-200 per month:
https://reddthat.com/post/8840079