Per gallon. Slightly cheaper here:

Home Page > Search > Q1030004 > ES#2522045 Anti-Freeze / Coolant - Priced Each - Q1030004

They also sell the M/B Citric Acid cleaner for the complete system flush. This is the same cleaner that is used in the service department of the Mercedes-Benz dealership when a cooling system flush is being performed:

http://www.ecstuning.com/Search/Citric_Acid/ES1609953/

From section 20 of the M/B service manual....This has been referred to as the "Gold Standard" of radiator care!
20-015 tells you to:

Drain the cooling system of all the old coolant
Remove the thermostat and replace it with a "forcibly-opened" thermostat p/n 000 589 63 00
Connect a flushing connection pipe (a tee) p/n 117 589 00 90 00 between the upper radiator hose and the radiator
Connect a tap water hose to the tool fitting
Open the surge tank cap
Set the heater to defrost on cars with automatic CC so the aux coolant pump runs
Start the engine and run it at 2500 RPM
Open all drain plugs and turn on the tap water to maintain a full cooling system at all times while the engine is runniing
Flush for 5 minutes to remove all old coolant
Shut off engine, shut off tap water, and allow cooling system to fully drain
Install radiator and crankcase drain plugs
Dissolve citric acid powder in 5 liters of water
Fill cooling system with solution and top off with water, bleeding as necessary
Install surge tank cap and start engine
Run engine for 15 minutes at approximately 2500 RPM and blip throttle occasionally
Make certain that coolant is flowing through radiator and heater core.
Open cooling system drains again and turn on tap water to flush cleaning solution from cooling system
Start engine and run at 2500 RPM for 5 minutes to flush all cleaning solution from system
Shut off engine and remove special tool from upper radiator fitting
Reinstall normal thermostat
Make sure that all drain plugs are installed and tight
Remove the surge tank, flush it separately, and reinstall it

The conversion then is 1.1 lbs of citric acid to 1.3 gallons of water to mix it up initially... then you are adding water on top of that (not distilled water as this is just a flush). So it's really 1.1 lbs per the entire cooling capacity of the engine which is roughly 9.5 quarts (around 2.4 gallons).

In summation...

flush old cooling system with garden house and kit for 5 minutes to get all residue out

mix 1.1 lbs citric acid powder to 1.3 gallons of water and add to engine, adding water on top of that until cooling system is full

Run for 15 minutes at 2500 RPM

drain

flush with engine running at 2500 with hose running and attached with drains wide open

Seal everything up and refill with coolant and distilled water

Another view...

www.w124performance.com/docs/mb/OM60X/citric_acid_flush.pdf