I was wrong. That's my runner up. $650 on the Bride of Frankenstein and Monster PF duo + base. I often forget I own them, as they're both packed away and not currently displayed.
I don't have it yet, but my most expensive item will be the Darth Vader bust ($1290 shipped).
Too date the most I've spent on an item is for the Hellboy II Samaritan Ex; $400.
If you consider assembling smaller pieces together, my Kinnikuman figures would actually take the top spot as I probably spent around $2,000 completing my set of them.