Did you confront the dealer about the insurance 
amount yet and get an answer?  I've had to deal 
with insurance before, and it can be messy to 
get a claim filed (you as the buyer are the one 
who must file the claim)  But I've never had a 
problem with the insurance amount like that.  
Both cels I had to file claims for were insured 
for the full value, and Anime Gambit (who 
packaged the cel very well but it looked like 
someone tried to spear it with a sword) even 
included the amount I paid for shipping in the 
insurance value.

I would first find out what they say about the 
amount.  It could have been an honest mistake.  
If not, and this is something they do regularly, 
well, I for one would like to know who this is 
so I can avoid them if possible.