Depends on what you want to put inside and the cost will depend on whether you want higher end equipment or low end equipment.
If you want to keep it low cost, then use a hang on filter, a clip on fan to keep the water cool, a cheap china brand skimmer and PL lights. For this type of setup, you could probably keep some zoas, mushrooms and fishes.
You may want to look at some of the tanks in the nano section to see how people set up their nano tanks.