Agencies pay per-seat and per-client fees that scale with their success. Self-hosted, pay-once tools replace per-seat pricing with a flat cost that margins actually survive.
The tool this job usually means renting is Eventbrite, at 2.7% + $0.79/ticket — roughly $665 a year, every year. Eventcraft does the job for a one-time $39 — running on infrastructure you control.