Actually, FY cels have gone up over the last 6 -12 mos...
I bought my favorite Miaka and Tamahome headshots for $100 and $80 respectively...they are from TV series.  Only nice OVA cels are cheap usually, because in most people think
they sucked- I have to admit that I didn't like them too 
well either, except for some of the 2nd OVA. I can't believe that people are paying $100   for oav cels personally.  The art for them is very good though. 
Also, there are lots of those, so yes, supply and demand is
at work there.  But good TV cels of Noriko, Tamahome, the twins, and Hotohori are getting more expensive all the time,
and lately cels of Chiriko.  Taro does have some really
nice cels at good prices.  It can be hit or miss.