Lee
I live in a hard water area too (tap water in excess of 350ppm minimum), my RO has been running for over a year now and I haven't even had to change the pre-filters yet, the only thing I change is DI resin approx every 5 - 6 weeks. If you are regular with your back flushing you should not need to soften the water first, I think that would cost you more than DI resin and new pre filters once a year.
Marc