Simplest and cheapest way to convert it would be;
10-12lbs of live rock ($40-$60),
20lb. bag of live sand ($20),
A few small powerheads ($15-25each) that move around 260gph,
Of course you'll need salt water, premixed is usually between $0.75 to $1 per gallon and you'll need around 10-12 gallons($8-$12)
I'm assuming you already have a heater if you we're running a fresh tank and you'll want to keep the water around 78 degrees F
All together the new stuff you'll need should run you between $100-$175 depending on where you live and sales/used stuff you can find.
The rock and sand will take care of your basic filter needs as long as you do fairly regular water changes and don't over feed. If you were using a HOB
filter on the fresh tank you can use it on the salt as well, it can help with water clarity and provide more water flow and oxidation.
Hopes this helps