From Porte Maillot to the Montmartre area during the day, my guess would be more like 10-12 €.
Using the Metro from PORTE MAILLOT would require taking the #1 Metro ligne (heading in the direction of Vincennes) to CONCORDE, then switching to the #12 Metro ligne (heading in the direction of Porte de la Chapelle) most likely to either the ABBESSES-Funicular or LAMARCK CAULAINCOURT stations as the closest to Sacré Coeur.
Unfortunately, the LAMARACK CAULAINCOURT Metro station is CLOSED for repair/renovation until approx June 16th....so the closest you may be able to get on the #12 ligne is ABBESSES-Funicular. The metro trip from Porte Maillot to Abbesses should take approx 20-22 mins (including wait-time & ligne transfer)