if you use linode, aws and custom backups (not the paid linode backup service) you can run a mastodon instance for less than US$20 per month ($10 for linode and ~$7 for S3)
source: computer fairies. we have over 200 users and federate with almost 2000 other instances and we're running comfortably
anything more expensive than that is for unnecessary features such as elasticsearch. you don't _need_ to pay more than this to run mastodon.