Try and understand the reasoning behind the RO/DI water. You need plain water - no hard minerals or other nutrients at all. For a tank that small you can probably buy 15 gallons or so at a fish store. Some well water is nearly that pure- if you are on well water than sure use your water. If you're using city water then there is no way you can run that through something like a carbon brita filter like the do for drinking water and think that's going to work. Once you have virtually zero solid in your water then you add the salts you purchase and they will include sufficient levels of other substances to make it a suitable saltwater tank. In fact there are salt mixes available you can mix your tank exactly like the salt in a particular sea. It's pretty exacting work and I don't think the technology has gotten any better I just think the salt aquariasts have gotten less snobby about it and are accepting of Nano Reefs and therefore the manufacturers are making their equipment smaller and less expensive.
Write down everything you think you will need and even everything you don't need and add up the costs. You have the cost of live rock, the cost of the the corals themselves (minimum $25 for an ordinary frag of 2-4 cells up to thousands of dollars for a big coral or unique one.
I am doing a semi nano one right now - 32 gallon Biocube and I don't think I'm saving any money but it will look pretty than just throwing crappy old aquariums together. In one package for about $500 US you get the tank, the sump/area and the lights. The way the sump is set up is that there is sp;ill=over from the aquarium through the holes in the back of the tank and the water falls through what ever types of filtration you want to put in there. An you can by fluffy pillow making stuff to use as a floss filter, all kinds of sponge material that you can cut up and use as filter material, you can buy your own mesh bags and a jar of whatever chemical filtration you want to use like
urigen, There is room enough in Chamber 1 to add a small protein filter (it's about $55) Chamber 2 is where your filter material goes and then in Chamber 3 you cranm a small 150 watt heater. I bought my RODI system for $159.00 because it's portable with an adaptor (like the kind you use for the Python) you can attach it to most kitchen sinks and let it run - mine gets like 100 gallons of water A DAY, I'll store my water in Rubbermaid BRUTE trashcans - plain pure water for topping off in one can and saltwater in the other can. You can probably carry your water by bucket - I'm putting my trash cans on rollers so I don't have to kill my old back carrying enough water - I'll just roll it down the hall (we're already using this system with our freshwater tanks and its 100% easier. I prefer to keep the salt and pure water on hand so my kitchen sinlk isn't tied up so much.
Also I decided that I needed a stand for my biocube - thats about $150.
I also decided I wanted a UV water cleaner in chamber 3 which isn't quite big enough for both the UV device and a thermometer and the pump = so I spent a ridiculous $70 to spit my chamber 2 in half since the filter material didn't take up that much of my regular chamber 2 then put the heater in chamber 3 and the UV water cleaner in chamber 4
If you want REAL live rock I think it's about $300 per 10lbs. I'm using dry life rock that I spent about $100 on for 40lbs (I only used 30) I needed some superglue and coral putty, that's about $20 or so to stick all my rocks together, Oh I'm going to buy a wave maker for the corals that like high flows of water. Oh I bought all the test kits, and special phosphate lowering filter material
I'd my nano system has ended up costing me close to $3000 in odds and ends and I haven't bought any corals or fish yet. I think If I started over I could have economized better and saved about $1,000. But realistically don't think $250 is in the ballpark unless somebody just gives you all their stuff. Then you could finish it off for $250. Like in your instance - if your fish store sells saltwater you can buy it instead of making it. Supposedly only about a 25% water change each month is needed if you have good filters, I've never bought water I just knew I wasn't strong enough to carry it.
Even if you know you won't be able to do this right now. It's been a lot of fun for me planning things out (I lived on Amazon for about 3 months) - reading reviews, watching youtube videos for the best recommendations etc, Then add up what a small system realistically will cost you, But you will kill everything if you at least don't start out with the right kind of water, the right kind of salt and the equipment you'll need to measure and mix the salt mixture.
