I live in a flat as well and the only answer is to become very efficient at using as little water as possible whilst not compromising speed or quality.
Resin only will cost a fortune at 250ppm from the tap. I know, I've tried it at about 200 - 220ppm. What ever Gardiner's resin cost meter thing on his website says, at 250ppm you'll be lucky to get a few thousand litres from a bag.
Also, if you've had the meter less than a year or have been in the property less than a year, you can, by law, go back to paying a flat rate charge for water but don't mention you have a RO system.