usually what I do to get "fast" results is to
check the closed Ebay auctions.  That has the most
recent prices.  but, I will have to warn you . . .
Normally, if you are bidding on a primo cel from
Yahoo Japan and if it is a popular series, then
chances are that you will have to bid much higher
than the average Ebay price since there will be
people who want it and typically, they would have
to use a deputy service and tend to place higher
maximum bids (there are no "last second" snatches
like on Ebay which gives more time to fight).

Which series/cel are you referring to?  Perhaps we
can give you a "ball park" range on the forum?  

Star Phoenix