Going on another wc's experience who did a similar thing he was getting through 2 bags of resin a month. Our tds is 450. You may be looking to get 3 or 4 bags a month. In the first two months of use at that rate you could have brought a good ro system for the money you would have spent on resin. You'll still need to buy some resin but only a bag or two a year.
Simon.