Hard to say. It's definitely more expensive to send emails, yes. But the upside is well worth it IMO.
For example, some services allow you to send content at a specific time based on audience's timezone. This increases open rate.
Or you can choose to resend the same email to audience who haven't opened the first time.
This could boost sales.
I'm not sure RSS can achieve that (there might be solution I don't know about)