That's a bit steep. I've paid quite a bit, but never that much through them!
Maybe you could put a bead of silicon around the outside top of the battery housing to act as a seal?
Have you seen these?
http://www.maxamps.com/Lipo-MV5250-Pack.htm
You could fit a crap load of them in there!
I have a couple, but only because I won them!
Can you post a close up of the main connector and the buss which all the lipo's are connected too?