To keep a few saltwater fish, I'd estimate about $50-$150.
It's almost foolish to pass up on going with a deep sand bed, no matter what your long term goals are with the tank. A $5 bag of Southdown Sand (or any fine grain sand about the size of sugar granules) will be enough. Seed it with a cup of sand from an established live sand bed (from a LFS or another saltwater tank).
If you're only keeping fish, your lighting is fine. If you want to keep live rock with encrusting coraline algae and a few low-light soft corals (you eventually will), then you'll need some more light.
29" Plastic Lighthouse Compact Aquarium Light, 1 - 55 watt bulb
This hood is $80 and will set you up for keeping Live Rock and some soft corals.
Even if you're keeping only fish, it is highly recommended to throw in at least a few pounds of live rock. It's not cheap, but they are excellent biological filters and offer more fascinating microfauna life than you can imagine. 10 lbs or so will run $30-$60.
Whichever filter you are currently using will most likely be fine for saltwater. The sand bed and live rock will do a great deal of nitrification and denitrification filtration and the filter will add much needed water movement to make the live rock an effective filter.
A protein Skimmer is a helpful addition. Not mandatory, but helpful. It helps remove dissolved organic solids before they become algae-feeding nutrients. A small air driven counter current skimmer is still a great bet for a beginner and shouldn't run more than $50-$60.
Then there's the salt. A $15 bag will get you started, but I buy my salt by the bucket ($35 mail order plus ~$10 shipping).
A Hydrometer to measure salt content runs around $10.
(Optional) Test kits. Most freshwater test kits do not work with saltwater. It's a good idea to at least have the general NH3, NO2, NO3 and pH kits to have an idea of what water conditions are.
That's all I can think of right now!
