if you do a search under 722.6 transmission you'll find a widely agreed upon figure of fifity thousand miles. expect something in the four hundred dollar range, and make sure either a dealer or mercedes specialist shop works on the gearbox. you can also think about an oil analysis from the drained oil to get some detailed info on the box.
I think the latest edition of Mercedes Enthusiast Magazine has an article by LJK Setright of Mercedes transmission, I haven't read it yet.