My current rule of thumb (by all means not bullet-proof) is the estimated price shown on the Pinside machine profile plus 20%. I think most people quoting prices within the Pinside estimated price are either in areas with lower relative pinball demand or people have not kept up with recent "pinflation". Coincidentally, the Pinside estimated price plus 20% also better matches the Boston Pinball Ebay summary.

That puts $2000 as a reasonable price for a decent working Judge Dredd, which this machine apparently was not.

Bargains usually are eventually discovered and cease becoming bargains.
http://gameroomblog.com/guides/the-top-ten-best-bargains-in-pinball-today

I'm not sure what is so magical about the $1400 number, but strange that this post used the same number, but to a different point: http://pinside.com/pinball/forum/topic/jd-is-not-a-1400-pinball

Ditto. I didn't know this existed aside from the original L1.

There are other places, but here's one. Remember that Judge Dredd needs nine balls!
ebay.com link: link