Hi,
IMO

It is entirely up to you what you like, and it is 
not ALWAYS the case that an extreme close up is 
more expensive. 

 You have to consider a lot of factors first: 

The popularity or rarity of the anime/character 
or a combination of both, the quality of the cel 
and acetate, the series/artist(A,B,C, etc,), 
whether it is an OP (opening cel), has an 
original background or not, is an important scene 
in the anime, has special-fx, or has any other 
special appeal, etc., etc. 

Or it could be as simple as two collectors that 
want one those cels bad enough during an auction 
and fight to the bitter end and bitter--PRICE!

 In general, that said, close ups are MORE for 
good or bad. They are usually done by key 
animators. But ultimately, it depends on what you 
like personally. Having the head fully in a frame 
is not always what you get on screen anyway, so 
there is no reason to paint it in. If the 
character has a sexy-bod (male or female) close-
ups are not always more! 

 Production cels are part of a moving body of 
work, so when you say "portrait", unless it is a 
hanken, repro. or post production cel having the 
subject "posing" is only an illusion, a dramatic 
sequence, or a freeze frame leading to more 
movement.

If you want more information go to my link page 
here (cheezy self-promotion) it might help in 
some way :

http://e.rubberslug.com/gallery/links.asp

I hope that helps..

Good luck,

 E