errr, if i don't remember bad i have seen a fancel
of that image somewhere recently... and if it was
not that image definately it was almost the same...
I suppose the one you have is a real production
cel, if that's the case then you really have a
very nice and valuable cel that many people would
like to have, and if it's a fan cel then you can
give my compliments to the artist as it's the best
resembling to the original fan cel that i have
ever seen from the movie....(that also has it's value)

besides of this, i don't know what you mean with
valuable.... valuable in price? or sentimental
value? or maybe a mix of them?.

As for me, even i have what we could call
"valuable in price" cels i consider more important
the "sentimental value". I have many cels with
sentimental value and would be almost impossible
to tell which is the most valuable....

here you have one of them.... Madoka from Kimagure
Orange Road