I think they are headed up but one way to check, http://www.pinballprices.com (don't forget the 's'), new website that has over 550 sales (90% from past 24 months) from live auctions and ebay - searchable database - all verified with links to actual sales sites. Shameless plug for my new website. Best viewed on laptop or tablet but smart phone is ok. Thanks.
Added over 6 years ago: Update: A little self-promotion here, but to check out where prices really are over the last 24 months go to www.PinballPrices.com (with a 's' - NOT the other guys that have been down for 2 years). All the sales are verified and have a direct link to that sale! eBay is about half the database of 700 sales (updated DAILY) but the other half are online and onsite auction sales that I have pulled. Prices do not reflect sales within the "pinball community" - private sales that I have no way to actually verify.
The database is powered by AirTable and searchable in desktop view, scan-able in mobile. Let me know what you think and how I can improve the site.

Added over 6 years ago: Update: A little self-promotion here, but to check out where prices really are over the last 24 months go to www.PinballPrices.com (with a 's' - NOT the other guys that have been down for 2 years). All the sales are verified and have a direct link to that sale! eBay is about half the database of 700 sales (updated DAILY) but the other half are online and onsite auction sales that I have pulled. Prices do not reflect sales within the "pinball community" - private sales that I have no way to actually verify.

The database is powered by AirTable and searchable in desktop view, scan-able in mobile. Let me know what you think and how I can improve the site.