Might be worth having a look at this for sale thread.  I ended up buying this machine and it is exceptional! It might give you a bit of an idea as to what your machine may or may not be worth.

http://aussiepinball.com/index.php?topic=8942.0

I wouldn't recommend putting a ridiculous price on it to start with, you run the risk of just turning people off straight away.  Yes have a little bit of fat in the price but don't go silly.  People on the forum are typically easier to deal with and more realistic the some of the knobs you get on eBay and the like.

I cannot see this thread because I am a noob! Can you re-post by any chance? Working up to those 20 posts