I rent my RO unit from Culligan water. I pay $39.99 a month and they take care of my filters, and it has a TDS reader attached. I paid $200.00 to have them install it...that covered the 3g holding tank under the sink, the little faucet that is above, the 5 stage filters and the time and trouble to install. I have horrible well water and the cost of filters would break me, so this method works for me....the water is unlimited, I only pay the $39.99....so much better than hauling jugs.
A unit usually costs around $200.00, then the cost of filters and trying to use a makeshift holding tank...to me it's worth it, and it's hassle free. If you have good water, most folks buy their unit, but renting the unit like I do is one option, so I toss it out there.
Now to be realistic......
You can set up the tank on an $800.00 budget...IF you buy used as much as possible, and are very frugal finding deals....but you also have maintenance and test kits, so it isn't like you spend that amount of money and your done. Trying to cram the cost of fish in that budget is not going to work. Some fish cost around $80.00 (the most I personally ever paid was $120.00) the cheapest fish are around $16.00 unless you get damsels. The reason damsels are so cheap is because they are so mean they kill off everything in the tank you add later, and bite you drawing blood as they mature. Unless you want the really aggressive fish, I don't recommend them to anyone. They are beautiful little fish, but in SW, there are lots of nice beautiful little fish that won't kill it's tank mates. Corals...you can buy frags, but they are far from cheap and their cost varies.
The best plan is to get the tank up and running, then slowly build up your stock as you can afford it. You have to go slow anyway with a SW tank. The more space you put between adding critters the more stable your tank. That way you have time to shop to get what you really want, and read up on the critter so you will know how to keep it alive, that way you get something you really like instead of just filling the tank willy nilly just to get fish in it.
To go with a new set up (everything purchased new), that means the filter system, rock, lights (not the big fancy lights), sand, pumps, skimmer and power heads (just the basics) starts at around $1000.00 and increases from there, for example:
20g to 30g expect to pay $1000.00
40g to 90g increases to $2000.00
90g to 125g increases to $3000.00
If you go with corals, or want an anemone, you will need the big fancy lights that start at around $500.00 (new) and increases from there. Again, you can purchase used lights cheaper, but bulbs also cost. I paid $70.00 each for my Metal Halide lamps on my 48" unit, that took 2 10K, and 2 actinic bulbs at $30.00 each. So once a year I paid out $200.00 just to maintain the lights. So even if you purchase used lights, you will need the bulbs and going with used lights, plan to purchase them right away..
I am not trying to scare you, but you do need to understand what you are getting into. You have a STARTING budget of $800.00, if you are careful you can get set up, there are some great deals out there to be found...but that is not all inclusive when you add the cost of fish, test kits and other goodies you will want down the road.