Well, that's the hard part. If you start small, you'll want to upgrade, and upgrade, and upgrade again..lol
Tell ya what, look on craigslist at some complete setups for sale, and you'll get a good idea.
Almost everything on my tank is used, and I paid:
$700 for my 220g tank, stand, canopy
$300 for the lighting
$50 for the sump
$350 for the skimmer
$75 for the skimmer pump
$100 for my return pump ( pumps water from sump back into display tank)
$150 or so for my powerheads (water circulators)
$400 or so in live rock
$35 phosban reactor (reduces phosphates in the water)
$50 for filter socks (mechanical filtration..kinda like air filter on a car)
$200 reverse osmosis/deionization unti (aka ro/di - necessary for making your water)
$50?? 44g Brute trashcan for mixing water
$30 super cool moonlights..lol..not necessary but cool
That's about it, off the top of my head. That's just basic startup items. That's not including salt @ $60/bucket. (you can use cheaper salt for just a fish only tank)
RESEARCH RESEARCH RESEARCH and take your time.