Ok, I know this is a bit late, but I'm about to venture into saltwater, and to your cost question, I was looking to upgrade my 10 gallon, which already had everything( as long as you have live rocks, you don't need a filter, you need powerhead, but seeing as DS hate current, I would use a protein skimmer. New tank and everything, your looking at 450-500$, counting 2 Seahorses. But I honestly wouldn't do it unless you already have had a saltwater. You can research all you want, but in my experience only the real thing can prepare you. And a decent sized bucket of salt is 30$.