Costs of running your tank should be based on how much electricity you're using - you just need to figure out how much you're paying per kilowatt hour to get an idea - of course you have to count in the electricity for the pumps etc.
With a tank that size a 25-50 gallon weekly to biweekly water change would probably be best. Once the tank is set up, compaired to a smaller tank, it should give you a lot of wiggle room and be more stable.. doesnt mean it requires any less attention though.
You could certainly have corals if it came with MH
lighting - though having those on will increase your costs.. they're not needed if you're looking to do FOWLR
or fish only.
Its usually not recommended to use actual sea water because of the die off from bacteria, plankton, etc. Also, pollution is a concern, especially if water is collected near the shore. It may be ok to start out with, but you;d have to be careful doing water changes with it. Ideally synthetic sea water is probably best.
Good luck! I'd love to have a tank that size