Well MB dealers set their own prices, MB only sudjests a price. You have to be carefull with some of the keys, the newer cars don't have a normal "key" and they can cost over $100 to get another. Some models have more expensive keys then others.
As for the dealer cutting the key in the back I guess one can do that. But I was told the "real" ones are cut in Germany. MB makes keys hard to get and I think only a handfull of places in the world can cut them. I tried to get mine cut by a locksmith and no one wanted to touch it.

Also MB keys are steel not brass, you do not want to use a brass key in a MB.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200