Mid - upper $40's .... that's what I paid for my 03, exactly 26 months ago with about 10K mis.
Unfortunately, for owners, this would seem like the 'good deal' price on a nice 04 Mamba, fortunately for buyers the market is crap and half of the country is going into winter now, good time to buy Vipers, or any car.
I think anything over $47k would be too high for any 04 considering prices on sweet 06's, unless it had some valued mods ... wheels, power mods, or something.
This is why I know I'll have my Viper a loooong time. With it's market value now, I love waaay more than the $'s it's worth. Kind of a good feeling actually, I doing a few mods this winter, knowing I'm hanging onto it makes me justify the $ spending.
Good luck on the search, you'll get a good deal, as there are so many good deals to be had, tough part is just deciding what you want exactly .. this can be said for just about any used car purchase now.