Lance, I wouldn't sweat the .05 microns or whatever your Distiller person quoted. RODI is proven adequate for saltwater aquariums the world over. You watch your TDS meter to determine when the filters go bad, or you just rotate them on a schedule.
I didn't see a real good answer to your basic question - Cost per gallon vs. RO/DI from Culligan (wal-mart) in a jug. Unless you live in an uber expensive water municipality, you will save $$ depending on usage, the question is how much.
I am REALLY strapped for cash these days, so I took my trusty Excel spreadsheet and got busy.... the outcome will be different for you based on how your water municipality figures your bill and how much water you need monthly.
First figure out your usage. That was easy for me since I go to town once a week with all those darn jugs! I use 10G a week for top off, 20G a week for water change, plus 10G a week for drinking/cooking water. That's 40G a week. 20G at $.35 a gallon (fresh) plus 20G at $1 a gallon (pre-mixed salt). Or $7 + $20. So $27 a week plus gas/trouble to town (which is 10 miles one way to McKinney for me or 20 miles one way to nearest LFS for pre-mixed salt water).
Now figure out how much water it would take to equate this via a RODI system. The ratio is 5:1, so to make my 160G a month of water would take 800G of water (system uses 5 gallons of water, gets 1 good gallon, flushes 4 gallons down the sewer).
Now armed with that, I called my city. They bill in a weird way, so nevermind my calculation. All up, I get $12 a month in increased bill for water/sewer for 1,000G of increased usage. $144 a year there.
Couple that with the cost of filters. If you base it on Marc's online pricing, you will spend $98 a year in filters if you follow his recommended schedule. So, that's $98 + $144 for a total of $242 a year.
Now for salt. I need 20G a month for 12 months or 240G of mix. Red Sea is $58.99 shipped for 175G (I know it can be had cheaper locally), and that ends up being $.34/G or $81 a year. So far we are at $242 + $81 or $323 a year.
My original cost was $27 week (plus gas) or $1,296 a year based on 4 week month. Hmm.. $323 or $1,296 a year. Not much to think about there!!! This will of course vary for the number of gallons you need per week, whether you need salt, which salt you use, and how your city bills for it's water and sewer.
Initial startup cost was $300 for me, since I needed a system that could handle drinking water too (with pressure tank, valves, sink spigot, etc.) Another $200 or so, and I'll have two storage tanks in the garage for fresh/salt storage. Another $150 on top of that will automate the entire system. (rest easy Marc, that won't be plumbed directly into the sump
)
Based on the monthly cost numbers above, that's $81 a month I'll be saving in water. It will take a little over 3.5 months for the system to pay for itself based on $300 startup cost.
