Salt pool - a bag of salt at the beginning of summer, and cleaning it as it needs - ie with a scoop, and maybe a kreepy krawly thing.
The newer ones cost bugger all really.
Chlorine ones are bit more work because you need to get the chem levels right and pay for the chems too. But pool guy once a month or so will sort that for about $50 depending on the company.

In winter they have a blanket on (well, in WA it is law not sure about anywhere else) and the guy comes to clean them once every couple of months.

HTH