Boston Pinball is fine for a quick and dirty reference. But for a better comparison I'd look at recent completed (not just sold) eBay sales, Pinside ads/FS threads, Mr Pinball ads, etc. Some will say current asking prices don't matter, but I say they do because those are the only other current options. You can't go back in time and offer to buy a machine that is already sold. But you can use it as a reference. Just remember that a seller is not obligated to sell something at any price, even if the last 5 examples sold for the same amount. A seller can ask what he/she wants, and it will sell for what a buyer is willing to pay for it.
Back to TAFs though. I've sold 6 of them since last summer. One was sold using Buy It Now on eBay ($5950). Two were sold outside of eBay. One was sold to a repeat customer. Two went to re-sellers, one of which sold on eBay for $6800. So Boston Pinball is only going to pick up the $5950 and $6800 sales. My $5950 sale included free tech support and warranty on the boards, and netted me $5700 after fees.
Of course there are often cheaper TAFs on eBay. Some with two sentence descriptions and pictures apparently taken during tremors still bring upper $4k sometimes. I'd hope whoever is buying those types of machines is looking them over in person first and is willing to spend hours fixing a machine that a seller took all of 5 minutes to try and sell. But no doubt there is a diamond in the rough here and there...eBay, Craigslist (for you career RSS feeders out there), even Pinside occasionally. Oh and speaking of Pinside, one with moderate mansion wear apparently just sold for close to $5500 here. That said, I'm going to start a FS thread now...
Good luck with the search Tom. I don't know what an 8.5 is anymore but I'd say the ballpark is $4500 to $6000. If you're ever down my way, feel free to call and stop by.