Art is worth what each person will pay for it.

I have no particular interest in DBZ cels but say 
for a series I was collecting no matter HOW many 
episodes there were. If there were a particular 
scene or character or outfit that were of 
interest, I would pay whatever it takes for a 
cel. Or whatever I think its worth. That may be 
higher or lower than what the current market is.

Take Cowboy Bebop (not a particularly cheap 
series to begin with) but I like Faye Valentine 
in a particular outfit (the dealer outfit from 
Honky Tonk women) Given 2 comparable cels of her 
I would probably pay 50% more for one of her in 
that outfit - makes sense? Probably not to 
someone else.

Also cel prices often have little to do with the 
quality of the anime itself. Its more what 
collectors want or like in design.
Miyuki-chan in wonderland - wonderful art - 
really the show doesn't go anyway - but gimme a 
molestering Clamp Catwoman!
X2 Musik video? (2-5mins?). Still I'd pay a 
bundle for a Purple Eyed Hinoto. 
Wish Musik video? - Would fall over for a Koharu.

I guess what I am saying is what does not make 
sense to you will make sense to another collector 
who WANTS an item.

As for professional valuations - they are a bunch 
of arse. Even famous artworks get "passed in" at 
auction because sellers cannot recoup their 
investment.