It is probably obvious, but if the restoration job is 
excellent and unnoticable, the restoration probably will 
add to the value of the cel.  Shoddy or work that further 
damages the cel will not help.  If the cel is of great value 
or sentimental value, I would use a restoration lab like 
SR labs (srlabs.com)  They have a good reputation in 
the animation business for restoring American 
animation cels like Disney Studios works and Warner 
Brothers.  The cost of restoration can be pricey, 
generally at least $100 per cel even for small jobs.  I 
have had them separate pencil sketches and some 
stuck cel layers, and they have done a good job.  I am 
considering having them repair an Ah My Goddess cel 
that has some minor hair loss.  I would ask any restorer 
first before having them do any work.

Craig