Thats a pretty good deal for a 125 gallon tank. Saltwater is definatly more expensive than fresh for sure. As you figure, water, salt and electricity, and live rock if you buy it ( which I highly recommend) will be your highest prices. I would start out with a FOWLR
system - much more easier for a beginner to start with.
live rock ( needed to maintain ph, hiding places for fish, place for coral if you get it etc. Live - about $4-10lb ( and would recommend about 100-200# of this rock ( so anywhere from $400-$2000) median price seems to be $7 per pound, though you can start out with mostly 'dead rock' ( dry reef rock eg: marcorocks.com) and use a few good pounds of live. I got 20# off marco dry for .89#
Do not just et any old rock for SW
. It can not have any metals, other bad stuff, and needs to be made of certain materials to 'buffer' the water. Hense using live or dead rock seeded with live.
Some people also recommend a DSB
( deep sand bed) and some use a small layer of sand. Bags of sand usually run anywhere from $10-$30 a bag for 15# ( dont waste your money on 'live' sand, just get the dry argonite sand)
Salt is about $50 for a 150 gallon mix, depending on what you do. Figure about 10-20 % water change once a week ( 15-30 gallons) you're looking at about $50 every 5-10 weeks for salt.
If you go FOWLR
you dont need expensive lights with high wattage, so dont worry about that for now.
I would buy some good saltwater test kits ( about $5-50 depending on the kit) start with ammonia, nitrite, nitrate and PH
I'd certainly recommend cycling your tank to save a lot of money and hassle too. It takes 4-6 weeks usually for a saltwater tank to cycle. Dont put in any fish before then, otherwise theres more $'s down the drain!
Thats a good starting point, feel free to ask more q's!