Hi,
In my experience Fuel Polishing costs are based on quantities, I just did a quick Google for polishing rigs and found a pretty interesting site here
http://www.gulfcoastfilters.com/fuel_polishing.htm
The one I have had the most personal experience with was a unit running an Alfa Laval MAB 103 Separator and a Filter full of plates packed with Fielders earth as a filter medium.
It used to clean the most badly contaminated fuel very well but needed a lot of cleaning itself whilst in operation.
The Algae x things have been around in one form or another for the best part of 20 yrs. Some say that the enhanced benefits are smoke and mirrors others swear by them.
I will say one thing for those type units though not neccessarily that brand. I was working in a workshop in 1992 and a Customer was complaining about problems with choked filters etc. He wanted to try one of the Magnetic Units the company sold but wasn't convincved it would work. He and the Boss worked out a deal whereby he would pay full price for it and if it didn't work the boss would give him back double his money plus remove it and put everything back the way it was.
That guy became the greatest living advert for that little workshop.
I was on a De Vries Boat that used these (
http://www.fuelking.com/en/index.html )things on Mains and Gens I am not convinced that they actually did much.