15,000 ghost contacts — profiles you pay to store but rarely email. That's your List Tax: ~$3,000/yr of your Klaviyo bill buying storage, not sales.
And switching saves $4,248/yr outright.
No ghost contacts at these settings — you're reaching your whole list. Even so: Klaviyo bills $400/mo for it, Brevo $46/mo. Same sending, $4,248/yr difference.