For the general service it would cost an average of 3.5k to 4k depending on your city. For any repair related, the fares maybe higher as it includes spare cost, labour cost and not to forget the gst.
The Duke 390 has a running cost of Rs. 3.57 per kilometer and a monthly fuel cost of Rs. 5357, assuming a daily run of 50 kilometers and Petrol at Rs. 100 per liter.