I believe it's impossible to answer your question without having some idea where the leak is coming from.
Worst case involves blown head gasket, water pump; radiator.
I'm merely guessing that these could run from as low as $400 to about $1200.
If it's simply worn hoses, much less.
You need to find a skilled independent versed in MB repair and pay him an hours labor or so to diagnose the problem.
1991 300-SEL - Model 126
M103 - SOHC
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