Hiei recently went on auction (Mandarake) for a 
bit over 10000 yen, I haven't been looking but I 
would imagine 'portrait' Kurama cels to be over 
15000 yen at least. I am very sure I am quoting 
the extremely low end of the price spectrum too, 
since I rarely see anything that affordable ^^; 

Good luck in your search.