Developers rent API docs, monitoring, secrets and webhook plumbing by the month for side projects that earn nothing yet. Self-hosted pay-once utilities cost less than one month of the SaaS they replace.
The tool this job usually means renting is UptimeRobot, at $8/mo — roughly $96 a year, every year. Upwatch does the job for a one-time $99 — running on infrastructure you control.