Maybe I'm not following something here, but you say your first 2500 gallons is $120, and each 1000 after is another $100. With that, I'm not sure why you think it's going to cost you $4-700 to fill it in the first place. You'll probably hit the minimum threshold just with your normal household use, so consider the first $120 mandatory. After that you'd use, with just a single pass and a 4:1 ratio 1500 gallons or so to get your tank filled. You said above that's only $100 per 1000g over, so you'd only be out $150 to fill it. Where are you getting $400-700 from?
Also, don't forget you can use the waste RODI water for other things too to save usage. You can use it for watering the lawn/plants, cleaning dishes, cleaning clothes, etc. I would imagine if you got creative you could probably booster pump it to a washing machine or dishwasher even. Any of those uses offset the cost you would have from using more fresh tap water for whatever.
Anyway, just for giggles I did the math. Presuming you get a 1 : 2 :: clean : waste ratio, which is better than even a water saver cartridge would normally do, the math follows a nice sequence. After the first pass, you have a 1:3 ratio of clean : total water. After N passes, you will have a ratio that approaches 1:1 (which makes sense, since you are cleaning the dirty more and more). The ratio for a given number of passes (N) is in the form f(N-1) * 3 + 2^(N-1) : 3^N. f(N-1) is the left side of the ratio for one less pass. If we then solve these ratios against x for 320 gallons output, we see that with 1, 2, 3...10 passes you need to supply : 960, 576, 455, 399, 369, 351, 340, 333, 329, or 326 gallons, respectfully. Now granted this makes some serious presumptions, like that you can maintain 1:2 ratio. It also presumes that you can push sludge through a filter, because your TDS is going to be insane after even a few passes since you're concentrating it in each waste pass. Some people around here have posted their TDS at the tap and at the waste stream, after one pass 150 TDS tap water in comes out as 250 TDS waste water, as an example. In any case, doing any more than about 3-4 passes would be pretty fruitless since you're doing a lot of extra work for not a lot of gains. If your filter isn't close to that ratio, you can gain a few more passes, but not many. A normal 1:4 filter, without running the math, is probably going to run about 6 passes before the returns aren't worth it.
Still not sure I'd recommend this, but if you're willing to burn through some resin and a filter, I guess that's your decision.
