It can cost 2k to start but deffinately doesn't need to.

The biggest turn off to most unexperienced slatwater keepers are algae outbrakes. Make sure you let the tank cycle before you add any fish or inverts. Things that help you get started....
Deffinately get live rock and live sand
DO NOT USE TAP WATER! use RO or DI or RODI water. (you can buy a RODI unit or buy distilled water)
Let the tank cycle for at least 2 months gradually increasing lighting.
Snails and hermit crabs are great just make sure you have extra shells in the tank for the hermit crabs or they will kill the snails for shells.
A protein skimmer isnt a must but I feel it helps alot.
A good size tank to start wold be 30-50gallons.
Think about what fish and inverts you want to put in the tank beforehand to make sure they are compatable.
I made the mistake of getting 2 black striped damsels that are extremely aggresive and have killed lots of fish (and I cannot take them out because they are in a 120g tank which would be impossible without taking all the rock out to get the fish). RESEARCH FISH AND INVERTS BEFORE BUYING.
A fantastic book is Marine Fishes by Scott W. Michael.
A sump and refugium will cost more but will only help the overall health of the tank.
Think about wether you want to keep just fish or also corals.
if you choose to also keep corals/anemones you will need to think about good lighting.
Do at least 25% water changes every month. 15%-25% every week or other week is best.
I will probably think of some other stuff later but if you have any questions feel free to PM me.

Heres a picture of my nano cube (12g). I only have one fish in this tank. I don't recommend a small tank (under 20g) to start even if it is cheaper, it is much harder to maintain.