You will need a tank, lighting depends on what types of coral you want to keep, a powerfilter and maybe a small minijet ph and a small heater.
then you will need salt, refractometer, thermometer, tests kits, sand, rock, stand, supplements, foods, etc.
You will probably be lookin at:
10-tank
15-filter
10-ph
15-heater
so about $50-75 for the setup
Then prob. another $300-400 for lighting and the mentioned supplies