most fish shops also sell ro water, some mix salt with it so its ready to go streight into a marine set-up not sure what thay charge for it but its a huge mark up on what it costs to produce it you must realise these marine set-ups costs thousands of pounds and need regular water changes 10% per week.
the trend is towards larger tanks ie 2oo gln plus and the people who own them are very wealthy [think footballers/popstars] thay dont even ask how much it costs to produce.
id be willing to bet a couple on here own one