For that money, youre unlikely to find a genuine factory item as they are horrendously expensive. There are some aftermarket ones available on eBay from time to time that are GRP but the quality and fit is unknown.
1500 sounds about right for a Maserati item..Sue you could get a company to make one for you.....If you find one for £400 snap it up as I've not every seen one under a grand.
There is a chap in France that posts on Maserati Life that is having a carbon fibre rear spoiler made to original design. http://www.maseratilife.com/forums/members/ahvlanico.html
He was looking for interest from US members and I suggested he posted here too, but he hasn't as yet.
The prices he was expecting from the manufactures sounded very good, can't find his post at the moment not on the home PC currently mobile!
Cheers
Mike