Assuming a default message size of ~10kbytes the SES pricing is about $6 for the connections plus the chunks. Your lambda pricing is negligible and the S3 pricing gets you another dollar or so.

But you're operating at the wrong scale-- 30k messages to a domain can happen in an hour by a spammer doing a directory attack. The point is that this solution is extremely open to abuse that happens all day every day over SMTP. Without a fixed-cost firewall to reject messages and only allow clean messages through to this solution, you are going to have escalating costs.