It's on going.
Yes you need enough for the initial cost, but you also need enough for maintenance.
Power heads can be anywhere from 10-60-80 dollars depending on the size, i recommend Koralia's.
Lights are really expensive depending on what you want in the tank:
-Fish only (any light will do, T8, T5, CF, Halides) can be... 60-1000 depending on how you go.
-JUST soft corals (T5(at least two tubes), CF, Halides) can be... 100/200- 1000
-all range of corals(T5(at least three tubes), CF, Halides, any combo of the three) can be... 500-2000, i have T5's on my 90 gallon and i'm loving them. you may be able to get away with Compact Fluorescents, but i don't recommend them for all corals, unless you got a smaller tank. (you may bea ble to get away with them as i said before)
T5 bulbs can be 25 dollars a peice, not sure how much halides or T8/CF's cost.
Live rock can cost a pretty penny, but price varies from place to place, and the quality of the rock. typically sold by the pound, so the more you want the more your gonna spend.
Sand, sand can be anywhere from 20-40-60 per bag depending on brand and weight. No need for live sand if you have live rock.
You may or may not need a skimmer, it's really down to opinion on smaller tanks. they can be cheap, to expensive depending on size. you're gonna need a refractometer. Don't waist time with a hydrometer. Refractos are so much more exact and IMHO much easier to deal with. refrcatos are usually around 60-100. A bucket of sand could be 40-60, but should last you around all year.
Get a python if you don't already have one, they make water changes a breeze.
What kind of filter do you have?
Get a ShamWow, lol.