The startup costs are for sure alot more than fw
but the ongoing costs do diminish over time. Just the salt and water you have to worry about and any extra electricity to power high lighting fixtures.
It's all what youre prepared to invest.
You could start a bio or nano cube at about 29g and see how that goes. Just alot more limiting as far as fish and corals and the water stability. I really really like my 125g and I have quite a few tanks. I spend the least amount of time on the 125g as far as maintaining it. My 4g pico took the majority of my time believe it or not. So bigger is usaully better as far as stability in a tank.
It cost me about 2k to setup my 125g and I even bought a used light fixture and tank and stand. :-(