this really depends on your aquarium size and if your doing a reef or FOWLR(fish only with live rock)
reef systems cost a lot more money then a regular FOWLR, so im going to give the suggest equipment for a FOWLR based on a 55 gallon
1-live sand or aragamax sand. live sand is suggest and will cost from 19-30 dollars per pag which usually consist of 15-25 pounds. aragamax sand isnt live but is still better then the rest, usually 20-25 dollars for 20-30 pounds. the amount needed varies on aquarium size, for example a 55 gallon marine aquarium should have 60-70 pounds of live or aragamax sand. $50
2-live rock. can be very expsenive depending where you buy from, i suggest liveaquaria. i have bought over 100 pounds of premimum fiji live rock for them only at 3 dollars per pound! stores usually sell 7-12 dollars per pound. amount needed varies on aquarium size, for example a 55 gallon marine aquarium should have 60-100 pounds of live rock. $220
3-filter. i suggest a canister filter or sump, for beginners i would go for a canister. the recommended canister filter for the best price is either the fluval series or the incredabily cheap penn plax cascade. this can range from 80-300 dollars for these brands so for a 55 it would be $130
4-heater. for marine aquariums i would got for the best brand possible as well as external control. what i use and recommend is the Azoo titanium heater 300w for a 55 gallon cost about $36
5-power head. i suggest the hydor koralia brands or ones alike this can cost about $50
6-protein skimmers. are optional but necessary, they can be very expsenive but some are cheaper but still good such as the sea clone 100 which cost $75
7-all glass hood/canopy. these increase light penetration, decreases water evaporation. for a 55 with brackets it would cost $30
8-lighting. for a FOWLR you can use the standard t5 for coralline growth for a 55 this will only cost around $100-150.
9-magnet cleaner. $10
10-hydrometer. $6
11- marine salt. 200 gallons of marine salt cost $50 for instant ocean
12-make your own filter pads. i do this and i save so much money instead of buying replacements, it cost 7 dollars for 10 feet of filter padding! found on ebay
13-frozen food. if you want to keep healthy vivid marine fish you should feed live or close to live foods, cost about $9 dollars for about a few months worth
14- 5 gallon buckets for mixing your water cost about 2 dollars at home depot
over all $800 dollars for the minimum equipment