Hmmmm, good question and a tough one to answer. Personally, I'm more about quantity than quality... I'd rather take $600 and spend it on 100 figures instead of just 1. But that aside, I'm pretty sure the most I've ever spent on a single item for my "collection" is probably this:
(http://www.p3donline.com/collection/11_24_04/marvin_watercolor_s.jpg) (http://www.p3donline.com/collection/11_24_04/marvin_watercolor.jpg)
click to enlarge
This was an exclusive at the old Warner Bros. Studio Stores, I miss those so much. They had the best stuff.... If I remember correctly, it ran me about $850... but it's huge, and limited to 350 numbered pieces, and came framed in that nice wood frame. I've got some WB and Disney sericels, but I don't think any of them were much more than $450 - $500 each...