Well, cels from OVAs and specially movies are
ussually more expensive than cels from series as
their quality tend to be better and also because
for obvious reasons there are not as many of them
available.

About the plot itself, sorry, i watched the movie
so long ago that i don't remember well, but those
pretty girls that you have probably seen on yahoo
japan these days are like androids who were
designed originally to control some huge
mechas.... well, it's not much of an explanation
but it's all i can remember.... the art was very
well done.