For the average buyer, $6500 is a lot of money for any first gen W that isn't a Cutlass Convertible or a really nice Turbo Grand Prix/Turbo STE.
That being said, we're far beyond the point where blue book (or bank black book) values really line up for these cars. They're too obscure to have collector prices, so it really does come down to what the buyer is willing to pay. Nostalgia factor is a big thing, and I can understand. There's entirely too much money bolted to and under the hood of my '01 Regal. Is an '01 Regal worth dumping thousands of dollars into? No. But one of my earliest cars was a '98 Regal, and I had the high school nostalgia hit and I wanted to build the car I aspired to have as a teenager.
That being said - For me personally, if I wanted a 94-96 GTP and was entertaining that price, it would have to be SO incredibly mint. Like nearly showroom fresh. And also personally, I'd much rather have a '96 so it has OBD-II and the slightly improved engine (better intakes on the 96-97 LQ1).