I'm pretty new to the SW world but it has been my experience that after the tank is established, maintenace is a little more difficult than
FW. It is also a little more expensive. You have to buy salt mix, supplements, purified water, and pay for higher energy use. The mechanical filtration needs to be cleaned more often, and water quality tests need to be done regularly. Changing water is a MUCH more involved process. Marine animals tend to be more sensitive so more care has to be put into the water prep.
As for a monthly cost, I really don't know. I buy in bulk so every 6 months I spend 100 - 200 on supplies, Just guessing. Salt is my biggest cost at something like 30 bucks a bag. The start up cost is definitely what hits the check book REAL hard.