You know, Im a big Nuriko collector.  He's been 
my favorite anime character for going on 5 years 
+ now.  The first cel I got of him was 280$, and 
at that time that was considered high as a cat's 
back.  Now the same cel'd go for about 600-700$, 
maybe even more.  Im not sure what the sudden 
rush on this character is.  The death cel on Ebay 
right now that's up to 500$....a cel just like it 
went for only 300$ last year.  In the time of 
about 4 months it seemed like his prices just 
skyrocketed out of nowhere.  Im probably 
partially to blame for that, but remember, I was 
having to beat out other collectors too, so I 
wasnt the only one offering or paying high amts 
for this character.  Anyways, the top three 
reasons I think he's more expensive:

1) He's only in half of the TV series, making 
cels of him harder to find.

2) He was a very heroic and tragic character, 
which appeals to many fans, therefor making them 
run wild for the cels, etc. This then creates a 
vast shortage of merchandise featuring him, so, 
prices go up, and up and up.

3) He's a bishounen!  We all know pretty guys go 
for oodles of money! ^_^

And...he's not even the most expensive of FY 
charas...Hotohori is by FAR the most sought after 
and highly priced of FY charas.  Here's one of my 
favorite cels of Nuriko, and it ties nicely with 
the post above from Shura, haha!  I paid what I 
thought was a ludacrous amt for this cel...but 
then on Ebay one from this same scene went for 
1500$!!!!  OUCH!