$71K for a 2002 GTS without stripes (stripes are a $3000 option) is a decent sale price. But a 2001 should go for $4-5K less. It doesn't sound like your dealer wants your business; take it elsewhere. As far as the trade-in on your Z06, if GM is still offering 0 percent financing on new Corvettes, that can't help the resale value on your 01. And you always take a major hit in depreciation on a one year old car.
Traditionally, the Viper has enjoyed a pretty decent resale value. I would speculate that 3 years from now, the GTS will retain a high resale value. This is based on the fact that GTS production ends this summer, and, at this time, DC claims they have no intentions of building a Gen III coupe. Even if they change their mind, the earliest model year we'll probably see one is 2006. So in 2005, the coupes will have been out of production for several years (the Gen III Viper is a convertible), and there will have been less than 10,000 of them built 1996-2002. I also believe that many potential buyers will be more attracted to the muscular lines of the Gen II coupes and the classic RT/10 than the styling of the Gen III SRT/10. So that should help keep the resale value up.
There may be a short dip in resale value this fall and winter as buyers of the 2003 SRT/10 dump their current Vipers and the preowned Viper market gets flooded. But after a year, things should level off and the resale value should creep back up.
There are, of course, other scenarios in the next few years that could affect resale: (1) the economy fails to recover and tanks again, (2) gas prices skyrocket and it becomes socially unacceptable to own a high-horsepower gas-guzzler, and (3) DC's German management decides that the Viper is not profitable enough and/or represents a threat to their Mercedes sports car market, so they decide to stop Viper production. Whether any of these will occur is yet another topic for speculation which I won't address.