If this is your first try with saltwater, I'd start with a 46G or even a 55G tank. Water stabilization & parameters are much easier to maintain as opposed to a 20G tank. Smaller tanks = high maintenance; especially for a beginner.
You really can't put a set price on a salt water setup. So much is in factor here - proper equipment, LR/LS, QT tank setup, livestock, salt water mix, test kits, variety of fish food, etc ... This is an expensive hobby that can run you hundreds, if not , thousands of dollars.