That's a difficult one because I don't use RO in all of my tanks, but do plan to use filtered water in all of my tanks.
I buy RO from LFS @ £3.50 per container (20-25L), and use approximately 3 containers a week, mainly on the river tank, and to top up any evaporated water in other tanks. Assuming a 4 week month, I spend around £42 a month on RO to drop the GH down to 14, with no significant impact on the nitrate levels on the river tank.
The nitrate removal filter cartridge and water hardness removal cartridge were £35 each, plus delivery of £5.95. Information I've found indicates the filter will do 600-800L tap water. One chap said his did 600L, starting with nitrate levels of 50ppm, and taking them down to 0. I will be using low nitrate water in all tanks, and expect to use at least 1 filter a month. Individual replacement cartridges are £13.95.
I will not be using the hardness reduction filter cartridge as much, and estimate that I will be using it less than half as much as the nitrate cartridge. Costs are £35 a cartridge (no replacement filters).
So, assuming I wanted to use 1400L/month water with 0 nitrate, and 700L with 0GH, I would use 2 nitrate filters and 1 hardness reduction filter = £90 for first month, then £55 each month after that (including delivery costs for cartridges).
Assuming I wanted 1400L/month RO = £196 a month (not including cost of purchasing containers).
Obviously RO is pure, and the water I get through the filters is not. Also, I've not included the cost of attaching the filters to the water system, which for me was around £25 for a water change kit similar to the Python ones.#
However, if I was just planning on using the water in the river tanks, the calculations would look like this:-
RO water 300L/month = £42 a month = £504 per year.
Filtered water 300L/month = 3,600L/year = £200 per year
Assuming 1 nitrate filter cartridge and 4 replacement filters = £95
Assuming 3 hardness reduction filter cartridges = £105
Sorry the answer has been quite long and rambling. Basically, due to cost I wouldn't have considered using RO in all of my tanks. I also struggle transporting the large containers from the LFS, especially lifting them out of the boot of the car and getting them into the house. Using the recently purchased water change kit made me realise that I could use that for the filters, rather than attach the cartridges to the pipes under the sink, which made looking at reducing the nitrates in my tanks a more reasonable idea. I won't be getting the nitrates to 0 because the filters also impact the KH & pH, but anything less than the 40ppm in my tap water will be better for the fish.