Is 100 amp service much less expensive than 200 amp?

Probably not. The only differences will be the smaller box, less breakers, and probably lighter gage wire from the street. No one installs a 100 amp service any more. There is just too much current draw in a typical modern house.

The number of breakers will obviously be the same since the service has no bearing on the number of branch circuits.
The wire from the street (unless it is underground) is not the customer's responsibility, it is the power companies.
It's not so much the actual use, it's the demand load calculation that typically determines a 200A service is necessary. Even if it's not really necessary it is always a good idea.

450 is right, I can't remember the last 100A service, even replacement, that I did. The cost difference really is negligible in the scope of things.