I installed a GE Smart Water whole house filter that I bought at Home Depot for about $25. The hose connection fittings added a few dollars more to the cost.
The cartridge prices are two for $13 for the charcoal ones and somewhat less for the particulate ones.
For us full timers, cartridges last for two to four months depending on water quality .
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.