The prices are high because AMG has a huge and 
rabid following relative to the cels available. 
Here are some web page stats: 

AMG: 24,000 pages
TM: 86,000 pages
The Little Mermaid: 81,000 pages 

To put this in perspective, TM is current, has 
hundreds of hours of animation, is shown on TV, 
and aggressively promoted. AMG is poorly 
distributed (eg. still no DVD in English), 
predates the web, yet still garners nearly a 
third the pages. 

With all those Belldandy fanatics (count me in 
that group) willing to pay for a cel from a 
measly 4 hours worth of OVA   movie, the prices 
keep going up and up.  

- Chi