1994 e320 check enegine light please help!
 

I have a 1994 e420 that has 199,000 miles and the check enegine light has gone on and off for about 6 months i have taken it to mb dealer 3 times and each time the code was that the drop down cable close to the transmission needed to be replaced so we replaced it then when the check enegine light came on in march 2007 they said that cable needed to be adjusted now all of sudden the check enegine light comes on again and they say that the whole entire transmission needs to be replaced due to internal errors. the cost is 7500 for a new transmission at the mb dealer. is that the right price for a new transmission and do you think that was the correct diagnosis in the begining?????