Jeff made a good point about your location. If you're in a city or suburb, you'll get more cash (larger market). Assuming it's in primo condition, the other issue is the economy. That has the same effect as if you lived in a rural area - smaller market. I don't know of anyone who hasn't tightened the purse strings lately. Keep in mind, it's also a 50 year old (give or take) piece of equipment. The gear box could blow smoke tomorrow, and i doubt anyone would be surprised.
All that said, i've seen them go in St. Louis for a fairly wide range of prices. I've been curious about the Parks for years now and have kept my eyes open to see what they go for. Complete machines in "plug and play" condition seem to fetch between $250 to $500 - auctions, newspaper adds, craigslist. It's not really consistent. Locally, they seem to bring less than what you might see on ebay. I've seen some advertised at $600 or $650, but they don't seem to move at those prices.
Given the recent crunch in the economy, you might be nearer the middle than the upper end of the "normal" price range.
Good luck with it.