"B." writes:

A friend of mine bought a used pool heater on eBay for cheap, like
$125, and it was local so he picked it up. Those things are a few
hundred bucks even for a small one. Anyway, they're still using it
after 5 or 6 years. When pool heaters are used a lot, they could
develop problems that could cost more than $100 to fix,

I can attest the repair costs.
I do all my own repairs.
On my small $800 heater, I'm averaging about
$100 a year in repairs. So far I've replaced
the heater manifold, the pressure switch, the control
circuit board and the hose fittings.

I expect next year to have to replace the blower,
it's seriously rusted.

If you don't do the work yourself,
expect to pay at least double.