There wouldn't be a 100k service.
I think the services in the maintenance book go at 7,500 mile intervals.
Minor services occur at 15k, and major services go at 30k.
The only maintenance items that go on time, not miles, are brake fluid (every two years), and coolant (every 3 years).
Thus, your 30k, 60k, 90k, 120k should be the exact same price because you are changing the same fluids and filters, plugs, etc.
As the car ages, though, the only additional cost would be repairs / replacement of items that are worn out.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".