Just a self-serving post, but given the original title of this thread I am surprised no one has mentioned my site, Pinballprices.com You can check it out yourself, but I manually search and enter any verified pinball sale I can find, mostly Pinside and ebay but also many online auction sites. I estimate it only captures about 10% of all sales, but I think it's the best price guide out there - and it's FREE.
Let me know how I can improve it (nicely) and I'll get on it. Thanks.
Gday don. Just checked out the average price page which loads slowly (as you noted) I would have thought this was down to the html generation though not the size of the database? Might loads faster if you paged it.
Good luck with it, interesting project.