It depends how far you go and what kind of system you're after. A FO (fish only) setup isn't that dissimilar to freshwater other than the addition of salt to the water. FOWLR (Fish only with live rock) requires a bit more and if you go all the way and go for a Reef setup (FOWLR with sessile and motile inverts) it starts really getting expensive.
I've been converting a 52g tank into a reef system and spent £500 (before the tank cracked and I've had to put it on hold) so far and expect to spend upwards of £1000 after getting live rock and upgrading the lighting and that is excluding a lot of equipment which could make things better. There's an awful lot of stuff which is optional which can automate the system and make it more stable which you could easily spend thousands and thousands of pounds if you wanted to.
Then, on the flip-side, there's nano tanks, where you could get going for a few hundred pounds but the smaller the setup the harder it is to sustain good water quality and the more you'll have to keep a constant eye on it.