Migadu prices like this; all the mailboxes you want and just pay by daily sending limit. ($4/mo paid annually to send 100 mails a day, for example.) https://www.migadu.com/en/pricing.html

Or if you want to pay for one mailbox somewhere and then have customized forwarding, I found this thing called Forward Email in another thread that looks interesting and I want to try setting it up self-hosted: https://forwardemail.net/#/