How much does it cost for a 55 gallon saltwater aquarium?

It depends on what your trying to accomplish, what your going to be keeping in it, as well as the material of the tank itself. Glass or Acrylic. You can have a Fish Only system, FOWLER system, or a REEF system.

Reefs are the most expensive and will run around $3,000 for a 55 Gallon set up Makesure you focus mainly on 5 things in a reef Aquarium and they are...

Water Flow

Lighting

Protein Skimmer

Berlin Style Open Sump

Large amounts of live rock (1 - 5 pound/ Gallon)

FOWLER (Fish Only With Live Rock) Is cheaper and will run about $1,500 for a 55 gallon Set up. Use ½ - 1½ pound live rock/ gallon

Fish Only, can go many ways, you can have a cheap hang on back filter or canister filter (Not recommended) or a below the aquarium Wet/Dry filter (Best Option). For a 55 Gallon set up maybe around $600 -$800 without a protein skimmer, though it is BEST if you do get one.