Well, I can give you what I paid today for a very good condition 520 with some extras. Maybe it'll help you decide if you're getting a deal or not.
This machine was a 1997 model 510, upgraded to 520. Very nice condition, no rust or notable dings anywhere that I've noticed yet.
For $1500:
SS 520 system complete with all the tables and pro fence
All manuals and VHS(!) tapes
Belt sander and about 6 extra belts
About 6 different SS saw blades each with an arbor on it, and some 5/8" arbors
6" SS Dado set
Sears combo shaping blade thing
Sears some sorta medieval-looking rip blade or something
Dial indicating gauge
Universal lathe tool (the heavy one)
Biscuit joiner
Lift assist
Casters
The SS wooden tool carriage holder thing
A lot of other nicknacks and stuff I can't remember or know what it is because I just got it today
(missing) Miter gauge/hold down - picked one on ebay for $41 shipped. He was using it for the SS bandsaw and power station that he kept.
(missing) The safety stuff (push stick, pads, fence rider). No big deal, can make my own.
I thought it was a fair deal - not a ridiculously good deal, but a fair deal. Had a 510 sell in my area for same price, $1500, that came with bandsaw and jointer, but none of the other extras (well, casters), so it's a crapshoot really.
I'd say offer the max you'd be willing to pay for the setup. If the seller bites, well, you're paying what you'd be willing to pay, so it's a good deal, right?