I have a 1993 W124 and my experience might be helpful. My drivers seat leather was very mildly cracked, and I considered replacing the leather. I called the dealer and replacement leather was incredibly expensive, so much so that it wasn't even a consideration. The service guy at the dealer gave me the number of a shop in North Hollywood that makes replacement leather that, according to him, fits perfectly and is virtually identical in looks, quality, appearance to factory leather.
Their pricing for the seat bottom and back rest is $504.90 for the leather and $200.00 to install.
Use this as a price guideline to compare. If they are cheaper, see if they'll send it to you and you can install yourself or send to a local shop. Their info is:
8116 Lankershim Blvd.
North Hollywood, CA
In the end, my buddy who works for Mitsubishi Motors U.S.A. told me that most dealers who get lease returns on Diamante's with leather send the cars out to get redyed and they come back looking like new. Most cracks are superficial.
I found a body / paint shop that does this. They have to use a special dye for leather. It's sprayed on. They charged me $100.00 for one seat, and it came out looking perfect. Like brand new. So good that I think I'll do my front passenger seat so that it'll look just as good.
There is also a company on the internet, I think Leatherique.com, that sells very expensive leather car products and also sells the dye you can use for yourself and they also have a crack filler product. If you do a search, you may find it mentioned on this site. It may be in the detailing section. I think a guy redyed the leather on his steering wheel, and one guy redyed his gray leather after using Lexol preservative on the leather when it took the color off the leather.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".