I am looking at making my 75g freshwater tank into a saltwater tank. Right now I have a fluval 405 for my pump I have 1 powerhead and heater. What else would I need and around how much would it cost?

You will need live rock, about 1.5 lbs for every gallon of water. This will probably be the most expensive thing because it can be up to 6-7 dollars a lb. You will need sand to. You should get a protein skimmer (especially if you want corals) and maybe another powerhead. I also would highly recommend getting a sump but that is totally optional. If you want corals or certain invertebrates you will need special lighting, which can also be very expensive. Could end up costing a lot if you get good live rock and want the corals, maybe over $1000, but that also depends on where you but things.