It really depends on what you plan on doing with it. I got a 7gal bowfront for about $40, this included the hood. I bought a retrofit light kit for about $53, sand for $20, and rock for $1 per pound, added 13#. So far, $126.00. I have a used hang on filter, but you could get a cheap one for about $20.00. Tank has not finished cycle, so roughly $150.00
Now when you add corals or LR, that is where the money comes in. If you have a source for cheap corals or another tank, it won't be too bad. I have purchased frags for about $15 each from a guy in Cleveland, almost 2 hrs away, so if I added 8 frags, the total cost would run around $270 + tax.
Just rough numbers, prices will vary depending on what you buy and local prices.