What would it cost me to convert to a marine tank?

I have a 20 gallon tank I use for breeding but I've decided to stop breeding and I was think either make it a planted tank or convert to saltwater. But how much would it cost to have a 20 gallon saltwater tank? I have everything a FW would have (heater, filter, tank, hood, light, siphon, air pump) so how much am I looking at for converting it and then upkeep? How much extra work am I looking at? Is it worth it?