same here. Did a timeslot management app for beauty salons etc. Tons of features, never once looked at source or even database (C#+pgsql)

The hard part were actually the sales but I'm getting quite some, going physically(!) to places.

Hosting costs $50, revenue is ~1k/month, nothing special but its basically on autopilot.

I did have the app for 6 months for free on a first client where they would give me feedback bugs etc until i felt it was stable.

I don't even have a pricing page, i try to see how much i get can from each client, from $5/mo to $100/mo