livestock is where things really start to get out of control with money. FOr just a nice small 20 gallon setup, you save a bit of money on Love Rock (20-30lbs @ 100-150$) and you have the potential not to skim (just do 4 gallon water changes weekly, however this would almost make you get a water purifier RO/DI for at least $150ish)... so for a 20 gallon reef set up without livestock,
tank - $25
Glass Top - $10
lights - $150 (250 to do it right, mh/vho/pc combo)
Powerheads - $80
Heater - $20
Glass cleaner - $7
HOB filter - $22
Bad of sand - $20
Live rock - $150
~ $585
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To set up a refugium (reccomended)
5.5-10 gallon tank $12
PVC fitteings and glue - $35
Drill bit to drill hole in tank - $10 (ebay)
Chaetomorph - $5
Live rock - $30
Light - $40 (pc)
~ $130
Then if you want a skimmer and/or purifier
skimmer $150
Purifier $150
~ $300
Live stock for my 7gallon has been about $300 and im not finished so if you want a really nice looked reef i would at least triple that
Live stock $900
So to "finish" it i would estimate around $1900, but dont forget all the monthly cost associated with running the thing
You could likely do it a lot cheaper than this, but by the time you get halfway... your budget just kinda goes bye bye and you buy what you need/want anyway. So realistically that is would likely cost, even for a 20 gallon tank, the bigger you go the only real cost difference is Buy live rock and livestock, and potentially a higher electric bill (should only really get significant if you really start putting on some big lights.