ST TNG used to sell for $4500 5 years ago. I've never really liked the title until recently when I played Marco's (Pinfan). What attracted me to his machine were the mods, and I think for this title, mods do it justice and increase the value.
Check out his restoration here
http://aussiepinball.com/index.php?topic=4316.0This is the best example of a ST TNG I've ever seen.
These days, using Marco's as a high end pin, you may look at $4500. Typical eBay pins hover around $2700 - $3200.
$4000 will buy you a decent ST TNG !