Woody's top figure of around 130k sounds about right. if you went for special paint (e.g. Bianco Fuji) and carbon fibre everything you're probably talking around that figure for an MC Sportline GTS or MC Stradale.
There is of course the option to give your QP to a 3rd party, spend near enough 6 figures and get yourself a QP Estate (or Bellagio) but that's not through Maserati themselves. I would imagine sky is the limit if you took your Maserati to a 3rd party custom firm. Speaking of which, that reminds me of Koenig, they used to take your Testarossa, give it a crazy body kit, strap twin turbo's on it and take the roof off - the legendary Koenig convertible!
Any particular reason for posing the question Loz?