Average I'm seeing lately (minus the ebay "thinks it's a gold collector edition Steven Spielberg personal prototype" pricing) is about $2200-$3200 leaning towards the lower 2's. Like others said, 1800 is a steal if it's not a project and a topper's worth about 150-200.
T-rex motors were a problem with early rom versions, and fixed in 5.10 (off the top of my head), but if your gonna upgrade go 5.13 or Chad unofficial 6.0. What was happening was the motors moved so fast, they were hitting the limit switch, but not shutting off quick enough after hitting the switch, eating up the gears over time... so in a service bulletin (actually 1 for each motor) they were slowed down/pulsed to fix this.
I also don't understand the ball disappearing question...but there was also a service bulletin to bend the trex's teeth so he wouldn't miss it/drop it.
If you're on facebook, I started a JP machine specific group for buying/selling/modding/repair help/etc. https://www.facebook.com/groups/JurassicParkPinballOwners/
And of course there are several threads on here for Jurassic Park that gave me the idea; here are 2 of my favorites: