A number of threads I found in the diesel discussion forum relate findings that the Zerex G05 is the only non OEM coolant that is approved for MB, and if you find it on Valvoline's home page, you'll find it listed as Daimler approved, along with a number of other European makers approval.

Price wise, I've regularly found G05 anywhere from $8 to $15 per gallon, and the MB bottled stuff was ~$30 the last time I saw any and asked the price out of morbid curiosity.
(The $8/gal price was at a heavy equipment hydraulic shop that supplied all the local operators and repair shops. It might be worth a quick run through the phone book!)

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...