As I am finding out, no saltwater tank is cheap.. and the cheaper... the crummier. Be sure to look into anything and everything about the tank before making the decision. You will have to talk to people about certain lightings and RO
water. I went way over my head when I made the last minute decision to turn my 20 gallon into a saltwater, now on its way to a nano reef. Are you looking to add coral?
The good thing I discovered was you can go with basics and always add onto your tank the more you discover you'll want to do.
I also discovered that a tank should run about 6 months before planting any nems (anemones) in there... so make sure you get the right advice from the right place because some guy at Petco sold me an anemone 4 months too early... I had to drop alot of money to keep this thing alive, but he is doing pretty well now.
Also look into a water circulation pump... I didn't realize that the fish and plants are used to the rough waters. There is alot to consider lol but most of my knowledge was gained through this site, so take some time and read through other people's forums.