Mailgun has its pricing page here - Flexible Email Delivery & API Pricing Plans | Mailgun | Mailgun
For brevo you can refer to their documentation here - https://help.brevo.com/hc/en-us/articles/7924908994450-Send-transactional-emails-using-Brevo-SMTP
You’d need to have some knowledge of SMTP while setting up your self-hosted instance.