It also depends on just how popular the empress 
is, not merely screen time. For instance, Asuza, 
the emperor of Jurai (I've only seen the OAVs and 
don't plan on seeing any other TM incarnation), 
only shows up in ep13 towards the last half, yet 
cels of him had been cheap compared to other 
characters. Rarity alone does not determine the 
monetary value of a cel - in El Hazard: 
Magnificent World, Phantom Tribe members get less 
screen time than Ifurita, yet Ifurita cels are 
faaaaar more expensive... ^_^'

Good luck