Well the Both Eyes Open Xellos on Yahoo JP Sold 
for alot of $$$$ - I think the most I've seen for 
any cel including Lord of Nightmares ($ 700+)and 
Otome No Inori (Shrine Maidens)($650-750)

Link: 

http://page5.auctions.yahoo.co.jp/jp/auction/e7814
009

this Xellos went for 123,000 Yen
thats approx. $ 1000.00 US or close to it !

I think that its the most expensive cel I've seen 
of Slayers so far...

Ame