Just because of where I live and the apparently low amount of CO2 in the water DI costs for me are pretty low - plus I use the water for a reef too. If I am going to run the RO might as well drop the money on DI. $55 for 7.5 lb (to overfill 6 cartridges) lasts me at least 6 months and might even be longer now as I am slowing down my automatic water changes now that the fish are 6".
There is a way around the CO2 if you wanted to take out ammonia or that last trace of nitrate. Set up an ageing barrel for the RO on a high stand or in the rafters. It has an airstone but no heater. After 12-24 hours the extra CO2 is gone, then open a valve and gravity flow through DI to a storage barrel. If the TDS of the RO is low the resin should last long enough that the cost doesn't outweigh the benefit by much. Its hard to find data on how well RO membranes remove ammonia, but years ago there was a simply member here (RandalB) who was a filter expert, who said that RO should remove around 70% of ammonia. This should be easy to verify, but I can't test it since I don't have chloramine.