How expensive is it to own a salt water tank?

That depends, of course, on the size of the tank and what you put in it.

A small tank such as a nano tank or a pico tank, is not too expensive to keep up.

There are many corals you can have without spending an arm and a leg.

Also some people have fish only which in general is less expensive.

The salt water can be purchased or made at home.

You needs light which can be very expensive for LED units or much less reasonable if you purchase lights such as T5s .

It is possible to own a salt water tank and not spend a fortune keeping it up.