Depends on your location, use of the vehicle and agreement with the dealership.
If you purchase the car from a luxury sales yard then they sometimes use a third party company for car maintenance. (I'm not exactly sure if you want someone other than factory trained tech messing with your car).
However if you purchase the car from an authorized Maserati & Ferrari dealership, they generally have a service department attached to the dealership that will take care of all that. Sometimes you may also be able to purchase a maintenance plan from the dealership. Which is basically you paying a set amount for service over a set period (either miles or years), that usually works out to cheaper than just paying every time you take it in.
The best thing is to call your local dealerships and mechanic shops and tell them to give a straight quote, email if possible. If they ever tell you that 'it depends' they're full of bs, tell them the tires you want and thats all they need to know to give you a solid quote. If you can get it in an email on their letterhead then you have something to take with you to the dealership who should price match if its lower than theirs.
I've seen first hand how people with BMWs will take them to third party mechanics for service an maintenance and end up paying more than they would if they had taken it to the dealership. Remember than dealerships order much larger amounts of parts for your specific brand, therefore they are more likely to get the part required for your car for cheaper than a third party who may have to special order the part at single part cost.
I own a 05 QP. I have done three services at this time and one clutch replacement, as well as one full set of tires have been changed. I now have 19000 miles on the car. Yes, the service expenses are more than you would have with a Mercedes, BMW, Porsche or another high-end, high-volume manufacturer. The cost is less though than at Ferrari, Lamborghini, or another one of those super-high-end marques. I only have mine serviced at factory authorized service facilities, as there always is the one or two warranty issues they need to fix. Overall I did not have the car in for unscheduled issues and the car proved exceptionally reliable. Mind you I have put the 19000 miles on the car in the course of a little over 1.5 years. Service Tech thinks I have no problem with the car because it does get used a lot. Maseratis are high performance cars meant to be driven, not kept in the garage.
Anyway, my expenses were:
1. 6,500 mile service (oil change, lube job, general service items, including material and labor) $ 745.00
2. 12,500 mile service (lube, oil and filter change, general service items, plus CHANGE OF CLUTCH, CHANGE ALL TIRES) $ 9,800
3. 18000 mile service (first "big" service, changing all fluids, filters, ...) $ 2,700
So, all in all not too shocking, but more than you would pay with any other German or Domestic luxury brand. I am excited and elated that my QP has a perfect service record though and is always ready to go!