Before I summon an Uber, is there a way to know, more or less, how much it would it cost to go from Charles de Gaulle Airport to Montmarte Paris's 18th arrondissement?
There is a fixed fare from CDG to Paris right bank of the Seine. It is 45 € with Uber X, not less not more. This won't fluctuate if there is a traffic jam or if it takes 2 hours...
You can view the estimated fare on the app just before confirming the booking:
Alternatively you can use one of the many fare estimator websites out there, including uber's own one which says that the fare is 45 EUR with UserX or Green:
As it turns out, this is to date the flat fare from CDG into Paris.