The Maserati reputation from that era is poor in comparison to today, which will feed the second hand values.
Rusty unreliable biturbos with some less than favourable reviews by popular journalists hasn't helped !
By they time they got to the GT, they had ironed out the early problems, and produced a nice reliable ish... car but people had lost heart by then.
Road tests of the day pitched the Cup against the E36 M3, which tends to be mid to high teens now, although there are a couple being chanced in the low £30's
In this instance rarity doesn't help value, it is not that it is a sort after model of a well liked car. I would have said the GT was relatively unknown and the Cup is a limited edition of a car that people just don't know about.
Whereas everybody knows a Porsche 996 and they are well liked cars, and you can buy 996 headlights on ebay for £650 a pair !!!