sub get_price {
   my ($self, $price_type) = @_;
   croak "need PriceType" unless UNIVERSAL::isa($price_type, 'PriceType');
   my $sth = $self->sql_find_price;
   $sth->execute($self->id, $price_type->id);
   my ($price) = $sth->fetchrow_array;
   return $price;
}