i have an RO, and on a water meter,
water only costs about £3 per 1000lts you use.
with mine to make 1000lts of pure it uses 3000lts in total. costing around £9 on a metre.
resin wise, i still have a half a bag of resin left that i brought a year ago.
i change my resin when the water hits 001ppm-002ppm
my water starts at 450ppm and comes out at 12ppm before resin.
so with your water through a RO you might not even need resin!!!