Discourse can still run on a 1gb memory server, which is about $7 a month with DO/digital ocean.
If you’re sending less than 1,000 e-mails a month with mailgun then that is free. Alternative free smtp sender is Brevo.
With discourse hosting you have team support + discourse mail.
In addition you will have domain registration fees.
There are some other less expensive server options, believe Hetzner may be one of the lowest cost ones in Germany.
Probably about $2 u.s.d. a month might be the minimum but there may be even less expensive servers than that in Indonesia or The Philippines.