The A.D. Boivin Expert X rear susp runs damn near $1000, so add that to the blue book value. Which is:
1999 Ski-Doo MX Z 600
Suggested Retail Value $2835
I'd say your sitting preatty good, baring that its a good runner. I'd say with that suspension odds are this person loved the whoops or may have even raced it. Either way, I'm sure it was FAR from babied. Check it over thoroughly, check motor mounts, tunnel, belly pan for stress cracks & get a compression test done too!